I cant explain why you may not find many gay musicians, honestly, with all that I can and trying not to be offensive, could it have something to do with attention and the lack of that a bassist and drummer may get?
I opened for a gay band last Friday, Their name was skate night, from new Orleans. At first I thought the bar was a gay bar, then I realized they were just there because the band preforming was gay. Found out when I heard some lyrics!

they had a keyboard playe a singer and a synth type thing that did beats for them. Their music wasn't all that good sounding but the singer put on a great show.
To further derail this thread,
do gays all of a sudden decide they like the same sex and same sex only? Are they straight but somehow really like ONE PARTICULAR person of the opposite sex, this would make much more sense to me than coming out 100% same sex only.
Let's trynot to get too politcally correct guys, it can get really annoying