I think that Sirius was a very well rounded character - despite the fact that we've only heard of him in only the first book, but we've seen him in books 3 through 5 (it's a shame, really. It wasn't enough time for Harry to know him thoroughly). Anyway, I see him as a person who, ever since he sat at the stool at Hogwarts and was declared, "Gryffindor!", has been neglected by his family, which caused him to be very rebellious.
Yeah, no one knows what he was truly like in his school days (except during Snape's worst memory), but I think that even though girls were after him, he was trying to find the right kind of girl, whom never appeared in his life or if she probably did exist, she moved on when he got arrested (and so left him).
Sirius was one of my fave characters and he still is, but I find my self doubtful about him being gay though (I'm not much of a Sirius/Remus or anything, so yeah...).
Other than that, who is exactly human and doesn't really have any feelings at all? (I agree with you guys: Sirius was somehow full of passion *smiles*) In most of my fics I write him off as someone witty, sarcastic sometimes (but only to be funny), lovable, and sometimes I write him off as the best friend who gives advice when his friends need it (most of the times to James). Sometimes I find myself wondering why J.K. didn't give him someone to be with (I mean, Remus, the poor guy, had Tonks for a while...).