I'm the same way for the most part, except I'm a woman so there's less of a stigma there since unfortunately it's still considered fairly socially anomalous for a heterosexual man to have many gay male friends or openly identify with gay men. However, I do sometimes get the occasional joker asking why I care about gay issues if I'm not gay.
At any rate, like you, most of the writers, musicians, and artists I latched on to in my adolescence were either gay or bisexual and as a result, the concept of same-sex relationships was normalized for me pretty early on and I quickly developed an interest in queer history and issues. I don't ever recall a time where I thought homosexuality was unusual. In fact, it made a lot of sense according to what was then my comically naive, oversimplified view of human relationships (men understand each other more than they understand women, so why wouldn't they love each other? etc).
I've always related to men on a very deep level, emotionally and intellectually, which probably has a lot to do with it as well. I've been jokingly told that I'm "a gay man trapped in a woman's body" which I find obnoxious, but I do feel a strong degree of empathy and identification with gay men more than any other group, including other heterosexual women. Furthermore, while there are exceptions, I typically relate to gay narratives in fiction and elsewhere much more than the more traditional heterosexual narratives, despite that, in technical fact, the latter more closely resembles my lived experiences. I wish I had a definite explanation for why this is, but I don't. I can only come up with half-baked explanations, like the one I alluded to above: I strongly relate to men but I'm also attracted to them, hence I feel a kinship with gay men? I don't know. This probably sounds like a load of bosh, but it's a part of myself I don't fully understand so I think about it a lot and it's not something I often discuss with other people, so I thought it fitting for this thread.