While saying its a gay disease is a bit much,
I think there is a certain political correctness
of not saying most touched are gays and
thus we're not spending money in the
right amount in the gay community to
educate them.
PC is sometimes destructive.
Probably anal is a better vector for transmission than vaginal
and thus explains the higher incidence rate.
Greater promiscuity would explain another part,
Greater incidence of non-protected sex.
Greater incidence of drug use.
etc.
While there are bi's, most gays stay in the community
and since its the community with the higher incidence
rate, it kinds of perpetuates the "tradition"...