Here is one reason I can think of to keep sexual orientation out of Dwarf Fortress and it is probably the biggest one I can think of.
What are the percents going to be?
Whatever Toady's best guess is? Seriously, all of our other numbers are wrong. Especially the ones about human biology.
On the general topic of gay dwarves, I'm all for it, whenever sexual attraction becomes relevant. Before then, its a nice characterizing detail, but it would also be a placeholder that would eventually be demolished alongside the other relationship mechanics. So a waste of the Great Toad's time, like others have been saying.
I will say this: some of you seem to be having a hard time separating the way your real world cultures view homosexuality and the way DF civs would. There would be no need to add in gay pride parades or other such things because DF currently doesn't simulate societal change, and when it does the conflicts surrounding sexuality, gender, race, culture, class, and religion will all likely be handled through the same general systems. Second of all, dwarves live in a monarchy, which means their attitudes towards marriage are likely pragmatic rather than the modern idea of marriage as love. Gay or asexual dwarves would likely still marry and at least try to have kids if that was what their families needed (unless they don't care what happens to their familes, of course). That doesn't mean that gay marriage wouldn't be a thing, mind you. As anyone who's played Crusader Kings can attest, sometimes you like the current heir and want to keep family tree small*. Plus, being able to ignore gender when arranging marriages could make the formation of alliances (for landed nobles) and family ties (for everyone else) quite easy. Or on the flip side, if dwarves aren't ok with gay marriage, sexual orientation would still be relevant because homosexuals in a hetero relationship are less likely to have kids (for the obvious reason) which can help lead to all of those Fun succession conflicts.
*See, when you marry a second spouse of the opposite gender, sometimes you have kids with them. And those kids won't inherit the kingdom... unless your old spouse's kids were to die. And your spouse rates quite highly in terms of plot power when the plot is to kill your family members. Sure, staying single is an option, but spouses are useful in that they help you run the kingdom and can give you powerful allies.