Imagine a population of 10 heterosexual male, 10 heterosexual female, and 30 homosexuals (of any sex, since it doesn't matter). The proportion here is 2/5 heterosexual and 3/5 homosexual. Those 10 heterosexual couples then produce 10 children each of similar proportion, leaving you with 20 heterosexual males, 20 heterosexual females, and 60 homosexuals.
Okay, I can see the beginnings of a valid point there... but your math is off.
10 heterosexual couples , 30 homosexual indidivuals
Those 10 couples have 2 kids each, since we're assuming "normal" human behavior.
Now, we have
Adults: 10 male, 10 female
Children: 10 male, 10 female
Non-Breeding, Purely Homosexual: 15 male, 15 female (some don't have partners)
Now what happens when they grow up? We can't assume they all become hetero and couple off, can we? Because this race is 3:2 homosexual. And they can't breed with their parents, can they? So out of every 5, 3 become homosexual and 2 are added back into the breeding population.
Adults: 14 male, 14 female
Non-Breeding, Purely Homosexual: 21 male, 21 female
Since these children cannot breed with their parents without causing severe inbreeding, their choice of partners is smaller than the 2 out of 3 that it was. Making the breeding population slightly smaller than 2 out of every 3 individuals. It's not a big difference, but it is one, and over the generations will slowly reduce the breeding population to smaller and smaller portions, if one is trying to avoid inbreeding of closely related individuals.
Reguardless, none of this has to do with gay marriage. Why must people be focusing on this little comment as if it offended them? And trying to pick it apart with math and logic and all that when ultimately, if I'm wrong, it doesn't matter to the original intent of this thread, which was... if I'm not mistaken... gay marriage in the game.
Additionally, I know from experience that in Dwarf Fortress, if one makes a third caste, female, that does not give birth (litter size of 0) and takes up half the female population, the race will die off within a few generations. I tried making a sterile elf caste and they died from lack of babies... I did give them a max age, but that's besides the point.