I wrote a post a while ago describing a bit of the complexity of how sexual orientation, sexual identity labeling and sexual behavior interact and I stand by what I said then. Now I want to say a little more about how and why people come to identify with a label, both in real life and in slash.
First of all, I think it needs to be pointed out that the whole idea of identifying oneself as gay or bisexual is a relatively new concept. People have had homosexual sex forever, and at some times and in some places that behavior was approved of and expected and in others it was severely sanctioned and in still others it was not even talked about. But the concept that homosexuality is an aspect of who one is, as opposed to what one does, is a modern Western idea. Various places and times have had assorted gender-bending concepts, but I do think that's a somewhat different issue. Certainly it is often related, since people who had "third gender" or cross-gender labeling were often engaging in homosexual sex. Still, because it conflates gender role, gender identity and sexual behavior, none of those categories are truly equivalent to the modern Western concepts of "gay" or "lesbian."
Slash is written in an assortment of fandoms and the worlds they exist in vary greatly. If the fandom world is not ours, it's worth considering how close it is to ours in making decisions about whether to use sexual orientation labels for the characters at all. Would m/m lovers in a Galaxy Far Far Away perceive themselves as different from the heterosexual romantic couples we see on screen? How and why? Would those differences lead to a label of homosexual or bisexual or something else? Or no label at all? These are questions worth considering.
I write in X-Men, which presents a world very much like ours, that's occurring in "the not-so-distant future" and has the addition of the mutant sub-species. Those who write in HP are writing in a fandom that concerns itself with a hidden world within our own, which has the addition of magic. Other fandoms where I'm not familiar with the source text, but I get from the meta that they are much like our world with some change, include SGA and Buffy. In all of these cases, I think it behooves the author to think about how sexual orientation identity labeling works within our world, before writing how it affects the characters.
Not all men who have sex with men (MSM) in contemporary Western society identify as gay or homosexual. Some MSM identify as heterosexual, in fact, and see any same-sex behavior as sinful and a lapse on their part, not an expression of their identity. Some do identify as bisexual for a period, and others as a persistent identity. And some identify as gay, even those who have been in love with and married to women. The latter often go through a long and painful coming out process that includes a developing and changing sense of self before they choose and accept the label of gay or homosexual. I think it's important to note that choosing that label does not mean that they've never had sex with women, never been in love with a woman, never had a significant relationship with a woman. It doesn't negate their past. It's about who they are, not what they do.
I think part of the misunderstanding between gay people and bisexuals comes from the very common phenomenon of gay men and lesbians identifying as bisexual for a period of time during the coming out process. So if we overgeneralize we look at people with a persistent bisexual identity as only half-way out of the closet. We shouldn't do that and bisexual people are right to get offended when some of us do.
OTOH, a common misconception from the other side is that if we are sometimes attracted to people of the opposite sex, or have been in love with someone of the opposite sex, then we aren't "really" gay or lesbian, we're bisexual. A related, and equally mistaken, claim is that we only claim the label for political reasons. I'm an old style second wave feminist and do believe that the personal is political, but it's not just political. And for many of us it's a very personal thing and a core part of our personal identity.
I think that fandom has a disproportionate (relative to the general population) number of women who live heterosexual lives and identify as bisexual, so this latter misunderstanding abounds here, much more than others. These women often feel that if they identify as bisexual due to attraction, even though their behavior is exclusively or predominantly heterosexual, that gay men and lesbians - in slash, at least - should also so identify, if they have any attraction to or history with the opposite sex. And in slash we usually do have canonical heterosexual relationships to contend with.
I think it's a mistake to presume that someone who had a canonical heterosexual attraction or relationship (like Harry's to Ginny or Scott's to Jean) must come out as bisexual, if he develops or discovers feelings for men. That may be where he ends up, but it's not the only possibility, and the writer should in my opinion be open to other possibilities.
All analogies are flawed, of course, but race is not a bad analog as such things go. Mixed race people identify in a variety of ways in our society. There has to be room in our society for people of mixed parentage who identify as biracial (like Tiger Woods) and people who identify as black (like Barack Obama). Race is a social construct, not a biological one, but that doesn't make it less real.
Similarly, a lot of us identify as gay or lesbian even if we have mixed pasts or somewhat mixed attraction. We so identify because it's a central feature of who we are and how we relate sexually. Just because some people could have sex with a person of either gender doesn't mean they want to or that it has the same quality of centrality to who they are if they do. And generally that is a feeling that develops over time, sometimes with a stop on the way station of bisexuality.
I identify as a lesbian and have for decades now. For me, the sure and certain knowledge that I was going to live my life as a lesbian, that I wanted my lover or lovers to be women no matter what happened in my relationship at the time, is what led me to embrace that label. I still embrace it. I have fond memories of romantic and sexual relationships with men (perhaps I should say "boys" since we were so young). I often think men are attractive. I clearly don't have an aversion to men's bodies or to men having sex or I wouldn't be writing and reading slash. I'm always happy to hear about my male friends' sex lives. But I know that, for me, sex with a woman is profoundly different and better and more core to who I am than sex with a man could ever be. I am not alone in this. Many real people of both sexes (mutatis mutandum) feel similarly. Fictional people can as well.
Here's how my X2 version of Scott, who has come to identify as gay, explains it to Logan after Jean's "death":
"Do you still think you could change, forget about that part, like you said? Could you meet someone else, another woman? Fall in love?"
I shook my head. "No, I'm pretty sure I couldn't. Even if I did meet another woman, I wouldn't try to live like that, not again. It was a mistake, what Jean and I tried to do. An honest mistake, a loving mistake, but still a mistake. I've given it a lot of thought. It's not just about sex for me. I think I notice men more, am drawn to men more in lots of ways. But a whole lot of it is sex. You can't help what turns you on, and it's men's bodies that turn me on. I spent so much time fighting it, and I want to be true to myself now. It feels good not to fight it anymore. I can’t go back. Sex with men is the only kind I want. It's different, there’s more to it – for me, anyway – than there is with a woman. It's hard to explain. I don’t know if you could understand, really."
"I know the parts are different."
I laughed. "Yeah, and that's certainly a lot of it. But it's not all. As close as telepathy during sex made me feel with Jean, we weren't really sharing the experience, not in the same way as when I'm doing it with a man, even a man I didn't love or even know well. There's something about having sex with another guy... it's hard to explain.” Well, that’s what I said, but what I was thinking was that I didn’t know whether or not I ought to try to explain. I wasn’t accustomed to talking about my sexuality at all, not having done so for a long time. And it felt a little bit risky to discuss it with Logan, although I wasn’t sure why. I took a deep breath and continued. “There's a shared understanding that I just don't think men and women can have, or at least that I can't have with a woman. Can I be specific? Or is that going to make you uncomfortable if I talk about homosex?"
"Okay. Here's an example: I love giving head. And some of that is just being totally turned on by men's bodies. There's little that feels as good to me as a big, hard cock in my mouth, down my throat." I sneaked a sidelong glance at him, checking to see that he wasn't getting upset with the graphic description, but he looked interested and listening. "It's more than that, though,” I went on. "I have this really intense impression every time I do it. I find myself thinking 'I know what this feels like; I know what he's feeling.' And there's an erotic charge and a connection in that knowledge that I just don't think I could have with a woman."
"Well, that's true. I can see that there's something there with a man that there isn't with women," he said, not seeming disturbed by the conversation at all. "But there's something else with women that you can’t get with a man. Some mystery or something. I don't know how to say it. Do you know what I'm talking about?"
"Yes and no. I do know that, for many men, that sense of 'otherness' about women is profoundly erotic. It doesn't matter that you don't know how to express it, Logan," I added with a smile. "I've heard it enough. Much of Western literature is devoted to expressing just that." I shook my head. "I understand it, in a way. I can teach it, I can write about it, but I don't feel it. It doesn't touch me the way it does you, or most men." I sighed and continued. "I've thought a lot about this, since Jean was...lost. I loved her so much, but not having her I've thought more about what kind of love it was, what kind of feelings I've had. I did think sometimes that I would change, when Jean and I were together, but I don't think it now. We loved each other and that was no small thing. But sex, even with some guy I didn't know, didn't care about, wouldn't see again, was always more intense, more fulfilling, more... complete than it was with Jean." I smiled wryly at Logan. "So, I really am gay."
Bisexuality is not the only option for people with histories of sexual behavior with members of both sexes. I think we do our characters and ourselves a disservice if we make it the default rather than carefully considering what works with our concepts of the characters.