If you want a reason for members of the races to be homosexual then the most likely one to me, given your creation myth, is that some of the gods were. No doubt all the races were granted aspects of thier divine creator, be it strength, wisdom or really sharp teeth. Thus, given that there is no reason for there not to be gay, bisexual or asexual gods, then they could have created their children with these aspects.
If you want to include homosexuality in your campaign world then you might think about deciding on a few gay relationships among major NPC's. It may never come to being important to the campaign or even known to the players, but if its decided now then if it does become more relevant its more likely that it will seem part of the setting and not applied at the last minuite.
But also you could have a gay couple run the next tavern the players stay in. Again the players will most likely never notice that there happens to be two male patrons.
One last suggestion for including it in you game world again looks to there being gay gods. It would be a good reason for why two gods are such good allies, even perhaps it is common for there respective clergy to marry each other. Or perhaps two of the gods used to be in a homosexual relationship and then fell out, one of them may have changed there sex and sexuality out of spite.
Despite being gay myself I have never played a gay character, but I believe this I because when I roleplay I like indulge in escapism and play characters with traits that I don't have.