I really don't think any of the Marauders are homosexuals.
There was one sentence in one of the books which made me think Sirius liked to have the girls attention and he kind of relished it because he was a bit of a tease.
But I always assumed he never did anything about it (as far as we know so far) because he valued his friendship with the other Marauders more. i.e. that he did not want a girl to get in between his friendships, this is pretty average isn't it for teenagers.
I don't think JKR wants to comment on this sort of stuff really as it has no bearing whatsoever on the story line. I also cannot believe it will be introduced in the final instalment as she said herself she will not be introducing stuff for a conclusion more clarity on the story which has already developed.