You know, this is actually a really good point
+ many points to the author.
Now I, am vehemently pro gay-rights/marriage/everything. I'd like to think that a gay character wouldn't affect my decision to purchase a game, however in the case of a dating sim .ect it would, as for a game with a liner storyline including the main characters gay romance (boy or girl) I'm not as sure really, if it was optional it would definitely not impact my purchase. My line of argument is that I play games to be immersed and play as myself or a relatable character, I think a gay character would throw me out too much if it was a forced inclusion, however fi it founds it's way into a less serious game (read: Duke Nukem, mindless shooters .ect) it probably would have nearly as much an impact.
However what I will say is it will definitely throw out a lot of interest from more homophobic groups, a lot of controversy (unfortunately) and a general loss of sales thus making it unlikely, I prefer the Bioware/Maxis approach where its up to you, as this both incorporates the gay demographic, keeps immersion and wont send the homophobes flying at it with fury.