I imagine that the reason why so many people feel a homosexual character must be stated as such, while heterosexual characters need no such clarification, is because heterosexuality is viewed as "normal" while homosexuality is deemed "unusual" or at the very least, "different." This highlights our assumptions about people themselves. A lot of readers enter into any character description with a blank slate. What else can you do, after all? But that blank slate must have certain characteristics in order to be anything besides an amorphous blob. If it's human, it probably has a predictable set of appendages (two arms, two legs) and other "normal human" characteristics like a mouth, a nose, etc. It might also have attributes like skin tone and height/weight that are similar to that of the reader or, alternatively, seem to be "normal" and thus more likely to appear in a fictional representation.
This leads me to my larger point. Many readers feel that homosexuality must be stated, as opposed to heterosexuality, because heterosexuality fits into the representation that they're expecting. Whether that's because (1) the reader him/herself is heterosexual (hence, the expectation of similarity/homophily), (2) because heterosexuality is viewed (accurately or inaccurately -- statistics are somewhat inconsistent) as more common than homosexuality, or (3) because the reader holds a prejudice against homosexuality that results in it seeming "unusual" or "strange" regardless of (1) and (2), readers tend to expect heterosexuality and are surprised by
anything outside that "blank slate." I apologize if the comparison offends anyone, but just as a reader would be surprised by a different number of appendages, a different orientation also deviates from the template that the reader holds. (It's a much different deviation, but if it splits from a reader's expectations, similar mental adjustments have to be made to understand the nature of the character.) Unless, of course, it's thrown in your face that all of the characters in the community are three-armed or, as the more relevant example, that they are all homosexual (for the latter case, think of
Rent), in which case you have other problems: it's unfortunate that the homosexuality had to be thrown in people's faces at all, and by definition everyone in the community is somehow set apart from the "normal," "expected" society.
...Anyway. Sorry for the stream-of-consciousness mini-essay, but at the risk of starting an argument over sexuality -- and at the risk of stating something poorly in the midst of this 5:20 a.m. monologue and accidentally inflicting offense -- I wanted to share a few of my thoughts here. Maybe next time I'll actually post a bit of poetry, eh?