People who would participate in an urban discussion forum are generally people who appreciate urban settings and, if they are old and lucky enough, have made the move downtown.
Now let's look at downtown demographics. In Toronto, I would expect that maybe 25% of the male population who lives downtown is gay. Of the remaining 75%, many might be older and less tech savvy and don't participate in online discussions, many might have families of small children which eat up their time, and many come from immigrant communities where their confidence in English might lead them to stay away from discussing topics in writing.
In the US, the percentage of inner city educated males with computer access that are gay might be even higher. In a typical US inner city, I would estimate that 75% of men are black or latino and have a much lower access rate to computers; of the 25% who are left, I would not be surprised if, in some cities, nearly half, or 10% of the male total, are gay. Thus, I think you can see, if not conclusively, why gay men form a large percentage of the posters on urban discussion forums.
For me, the bigger question is why women don't participate in these kinds of fora.