penknife focussed on presumption of canonical heterosexuality in the reader or viewer. She pointed out that many readers will see certain behaviors exhibited in characters or see other behaviors not exhibited and see those as determinative of heterosexuality of orientation, when they aren't necessarily. I agree with her on that, but I'm a little more interested in the presumption that occurs in other places. I'd like to focus on three of those places here:
* the presumption of heterosexuality within the universe of the fandom, specifically within fandoms set in current North American society or in a society much like current North American society
* the presumption of heterosexuality within the character himself
* the presumption of heterosexuality (or the fear of exhibiting openly homosexual behavior or desire) in the producers of canon.
Note: Because I write m/m slash I'm using male pronouns and focussing on male homoerotic desire and male homosexual identity. Much of what I say applies to lesbian desire and identification, as well. The similarities and differences are subject for another time and place.
Presumption of Heterosexuality Within the Society of the Source Text
In those fandoms set in current society or in a society as heteronormative as our own, I think it's worthwhile for fanfic writers to think about how that presumption of heterosexuality affects our characters. I think that examining the effects on our characters and interpreting their behavior in the light of that examination helps us to understand them. We can, for example, reasonably conclude that there is pressure on some characters to behave in a heterosexual manner even if their core orientation is otherwise. Same sex desire in some of the canonical characters may be deeply hidden - hidden from the supposedly canonically heterosexual character's intimates, hidden from the narrator, if there is one, consequently hidden from the reader or viewer. People go a long way and work hard to suppress same-sex desire and hide same-sex behavior in a heteronormative society. Because of this, heterosexual orientation is not a character trait one can easily and accurately infer from behavior.
Not all people who experience same-sex desire hide it, of course. And of those who do hide it, many do in some venues but not others (e.g. out to close friends, closeted with family of origin and colleagues). Still plenty do hide fairly universally in real life and it's reasonable to me that some would in X-Men movieverse, for example, since it's our society "in the not too distant future." It's reasonable to me that some would in Man from UNCLE, since it's our society in the not too distant past. The closer the society is to the one we live in, with its presumptions of heterosexuality, the more likely it seems to me that some individuals will hide their homosexuality and appear canonically heterosexual.
With a character like Cyclops, whose canonical hallmark characteristics are an obsession with doing things the "right" way - with rules and order and fitting in - it's quite easy to see how same-sex desire would be hard for him to own and act on. I'm in no way saying that a gay Cyclops is the only possible interpretation, but I think it's one that can well be consistent with canon. He's got many of the personality characteristics common in deeply closeted men. He's also in a position where his close friends, the people he lives with, and his colleagues are all the same people, which makes the out-to-friends-but-not-at-work closet option a difficult one. And, in movieverse anyway, he is a teacher as well as a mutant superhero. Closeted high school teachers are certainly still a big part of the educational landscape.
I write Scott in a couple of different versions (X1 and X2) but in both of them he is a gay man who has lived a heterosexual life for a number of years. I think that's an interpretation consistent with canon, and one that allows for exploring and interpreting his canonical characteristics in a manner I find interesting. I hope my readers will, too.
Presumption of Heterosexuality Within the Individual Character
Beyond the presumption in society as a whole, there is the presumption of heterosexuality within the individual. For every gay man who feels like he always knew, there's another whose homoerotic desire came as a complete surprise to him.
Someone may engage in sexual relationships with opposite sex partners and be unaware of same-sex desire. He may reinterpret sexual feelings he has for other men in non-sexual ways (as admiration or competitiveness). The pressure he feels from society as a whole is applied both externally by social convention and internally, as the presumption of heterosexuality is incorporated into the sense of self. He may well not know that he has sexual feelings for a man - or may know but not realize how intense they are - until he is in a sexual situation with a man.
In fanfic the situation that brings about the realization is extra-canonical, but I don't think that makes his homosexual desire anti-canonical. In skillfully written "I never knew I was gay before" stories, he's still the same person he was in canon. Canon was the story of what happened before he found this out about himself. In fanfic, as in real life, it may take experiencing homosex for him to know what his orientation is. For some people it's the realization that homosex is so much better than the heterosexual experiences they had that leads to self-realization of their sexual orientation.
I think fanfic can convincingly be written where the guy just doesn't know what he liked until he tries it. That part is distinct from the situation described above, where the character is hiding his feelings, but the dichotomy of observable heterosexual behavior and homosexual desire present in fanfic but not observed in canon is common to both situations. I've never written one of this type of stories, but I've enjoyed reading them when they're done well and I've certainly heard real life coming out stories that meet this model.
Presumption of Heterosexuality Constraining Producers of Canon
A third area where the presumption of heterosexuality sometimes obtains is in a reticence by the creator of source text to exhibit homosexual behavior or desire, even when leaving it out makes canon less credible. One can look at a lot of books, tv shows of a certain time period, and comic books and wonder "What happened to all the gay people?" Bruce Bawer likens it to African Americans watching "The Andy Griffith Show" (popular television sit com of the 1960s set in a small town in the American South) and seeing no black faces at all. Segregation within communities was certainly commonplace at the time, but there just weren't southern towns like that, with absolutely no black people anywhere. Yet that's what was shown on TV.
Similarly, if one takes off one's heteronormative lenses and looks at an all-heterosexual fandom, something is clearly missing. Often that something is missing because the producers of canon feared that having gay characters would not be acceptable to the audience or to other parts of the society. Comics in particular have long had concerns about backlash to homosexual characters, or characters who might be thought homosexual (at least since publication of Wertham's Seduction of the Innocent in the 1950s).
Sometimes we witness the process by which the producers tentatively open the closet door, and then run back in (e.g. Marvel creating Northstar to be gay and to come out in Alpha Flight issue #50, slamming the closet door until he finally comes out in #106, and then after backlash just kind of forgetting about him being gay for the rest of the run of Alpha Flight). I think it's reasonable to assume that this happens other times when it's not visible to the readers. As fanfic writers we can make our universe more credible by populating it with some non-heterosexual characters, even if the producers of canon didn't show us that part. We can view that lack as reflective of their omission rather than a canonical stamp of heterosexuality on all canon characters. I think even stories that are not slash per se gain verisimilitude when some gay characters are included, even if only on the periphery.
I do think that we need to be careful how far we go in adding homosexual desire and behavior to characters. As I've said elsewhere, I'm very leery of the "Everyone's gay" phenomenon seen sometimes in slash. If too many characters in any particular group are written as gay, I lose my willing suspension of disbelief, unless it's a group that would be expected to have a high proportion of gay people (e.g. I would not think there are too many gay people in a fandom set in a gay social circle, like QAF). I do think that many, even most characters, have the potential to be written as experiencing homosexual desire and/or developing a sense of homosexual identity, if developed as such by a skillful writer. I think part of being a skillful writer is choosing carefully and not overdoing.
For all of the reasons mentioned above I think we err if we see "canonical heterosexuality" as something observable and easy to conclude from canonical behavior. Certainly it is not as observable or easy to conclude from behavior as Democratic party membership or preference for brussels sprouts, to use a couple of examples I've seen compared to canonical heterosexuality. A character may canonically prefer brussels sprouts over asparagus, and I as a reader or viewer have no reason to doubt that. Generally, if people eat brussels sprouts all the time, it's because they like them. OTOH if people project a heterosexual persona it may be because it's true to who they are, but it may not. It may be because they are hiding homosexual desire and/or behavior, have not yet discovered that they have homosexual desire, or because heteronormative conventions in the original medium led to homosexual desire and behavior being excised from the limited views of the characters that we see in canon.