I think that when people use terms like "lesbian," "gay," "bisexual," and "heterosexual," they often mean very different things by them. Identity politics (and even just identity claiming, without considering the politics) can be really complex. There are women who have a history of sex with both men and women who consider themselves to be bisexual and those with the same kind of history who consider themselves lesbians. There are similar divides among men living a homosexual life but with a history of sex with women as well as men. Part of it can depend on how they felt when living an outwardly straight life. But part of it is that there is more than one thing we're talking about here.
I think there are three distinct concepts that interrelate: sexual orientation, sexual behavior and sexual identity labeling. The three exist in different ways for different people and don't always line up in a straight, perfectly matched line. A woman can be strongly oriented towards sex and love with other women (the classic Kinsey 6) but have a history of heterosexual behavior. OTOH, she can be a Kinsey 3 or 4 and only have ever had sex with women. The two modalities of orientation and behavior intersect in different ways, depending upon the individual.
The third modality, identity, varies too. I've seen this discussion play out in so many places. One woman who is attracted to people of both sexes, who is in a longterm relationship with a woman, who has no intention of ever being sexually involved with men again, may call herself a lesbian. Another woman in exactly that position in the modalities of orientation and behavior may identify strongly as bisexual. IME men are less likely to identify as bisexual if living (and intending to continue to live) a homosexual life, but it does happen. It's up to individuals to craft their own identities that way, I think, within certain agreed upon limits. Of course we have such trouble agreeing on those limits!
I think it's true that in certain quarters of the lesbian community it's considered more "pure" or something to only have had sex with women (the term "platinum lesbian" is often used for that). I also think that being a Kinsey 6 is sometimes considered bragworthy. I think it's a little silly, but what do I know?
In writing slash, we're usually dealing with characters who don't have those three modalities lining up so neatly, at least not for all their lives. That's partly because we are working with canon in which our characters are often portrayed in heterosexual relationships. It's partly because what interests a lot of us about slash is exactly that mismatch, the sense that what's seen in public (heterosexual presumption and/or heterosexual behavior) is different from what occurs in the character's head and/or in the character's bed.
In my fiction the only character I write who lines up so neatly on all three modalities is Northstar. I portray him as a perfect Kinsey 6 in orientation and with only homosexual behavior and with a strong gay identification, as well. I do that only with him to reflect my own belief that there are very few people on that end of the scale in all three modalities. I chose Northstar to represent those few people in large part because he is gay in canon.
My Cyclops (both X1 and X2 versions) identifies strongly as gay. He's not a Kinsey 6 - he had a serious relationship with a woman and had strong romantic and sexual feelings for her. In my X1 version he talked himself into believing he was heterosexual and suppressed his homosexual feelings for a long time, thinking (hoping) he could do that forever. He therefore goes through a long and painful coming out process as he breaks up with Jean and claims a homosexual orientation and identity over time (painful for both of them). In my X2 version, he and Jean became involved recognizing up front that he is gay and deciding to try to make their relationship work in spite of that. In both versions he has some attraction to women, but a homosexual identity and orientation is a big part of his personality. In both versions he comes to feel that living a heterosexual life (behavior) was not being true to himself and is something he should not do and will not do again.
My Angel is in some ways the flip side of Cyclops. He strongly identifies as heterosexual and that's where his primary attraction is. But he has some attraction to men and finds he can enjoy sex with a man when he tries it. In the long run, though, it's not something he sees as a big part of his life and he does not consider himself bisexual. He's different from Cyclops in that he's considerably more comfortable with his sexual expression throughout. He is, of course, not under social pressure to enact and emphasize the part of his sexuality that's less core to his personality. Straight guys are lucky that way :-/.
Oliver, an OC of mine, is heterosexual in orientation and identity but had sex with men when it was expedient to do so (he worked in sex trade as a teenage throwaway). His homosexual behavior did not affect his heterosexual identification except to make him very sensitive to it, worried about being thought of as gay.
Heather Hudson (co-director of Alpha Flight) presents another POV in my fiction. She was a LUG - a lesbian until graduation. She thinks of herself as someone who "used to be a lesbian." She doesn't see herself as bisexual since she's living a heterosexual life and doesn't view her identity as being tied to her sexual orientation.
My Logan completely rejects all labels associated with sexual orientation, identity or behavior. He just wants what he wants (and whom he wants) and doesn't think much about what any of that means. Since for a long time he was so divorced from human society in some sense, he finds that the idea of claiming identity or group membership according to sexual orientation or behavior just feels alien to him. He says to Scott that he thinks it's like saying "what team you're on" and he's not much of a team player. As Oliver tries to explain to one of the other kids, Logan isn't gay or bi-, he's just Logan. Still, his orientation is bisexual even if he doesn't claim the identity that goes with that orientation. He is attracted to members of both sex and acknowledges (to himself, anyway) that he misses having sex with women sometimes.
IMO there are very few people who are on the tails of the orientation scale - the perfect Kinsey 6s and 0s. I think a lot more people think they are 0s than truly are because there is such a strong presumption of heterosexuality in our society that people often do not acknowledge or examine same sex attraction. A lot of what interests me about slash - both as a reader and a writer - is the interplay of the three modalities of orientation, behavior and identity.