This is an interesting thread...I definitely agree with the OP, drug and gay subcultures are linked, in some way. At the same time, though, illegal history aside, I don't think it's as true as it would have been when homosexuality was frowned upon by most of 'proper' society. I mean, hell, in America today, male homosexuality, at least, is very out in the open in the mainstream, on TV and in commercials, and it's pretty much the same for lesbians as well.
Transsexuals have to be a different story, though? They still aren't accepted by society by and large, in my thought, so I would consider their link to the drug subculture still alive and well - they're both basically forced into the underground of society, and not always by choice. Then again, I haven't had much experience with transsexuals myself, so I might be overstepping when I say that, but that's my perception from afar.
I've picked up weed before from a gay guy, and once somehow got tricked into going to what was basically an all guy, gay, party, lol, just to smoke a lot of weed, drink beer, and do some coke. It was very odd being the straight guy out when everyone else was high and horny, but there you are.
Also, I think this girl that I get weed from sometimes is almost definitely a lesbian, but I've never asked because I really don't give a fuck and I don't want to piss her off because she's such a great hook up.
But, regardless of society's acceptance, let's be honest, we (the people who are old enough to be on here) were basically all taught in school that society is founded on heterosexual families who might drink alcohol or smoke tobacco, but that's about it.
So, just the fact that gays and drug users, by doing what they like to do for pleasure, are basically directly subverting the same normative social structure that we were, for the most part, taught to, by default, follow as we ourselves became integrated into adult society, by itself leads to a relatively strong bond between our two subcultures. Also, in what I just said - the connecting link is a seeking out of pleasure. We have different preferences but hell if we just want our fucking fix.
Oh shit, I did forget one major thing...in the black community in the U.S., homosexuality is still very much frowned upon, so this link between drug and gay subcultures would be very pertinent to that community, much more so, than say, in 'liberal' white suburbia, to give an obvious example.