If I want realism, I'd go outside for a walk. I would not expect to find realism in the eponymous Dwarf Fortress, any more than I'd expect to see it in Hogwart's.
Orientation doesn't affect egg fertility AFAIK.
Oh? Is that new? Or have I been wasting my time the last few years? Many's the hen that went in the pot after two consecutive clutches that would not hatch.
Getting back to the original idea, I find it interesting, depending on how it is implemented. So long as I am not playing a dead civ, I could not care less whether they all are non-marrying. At least some of the migrants are working age adults, many with useful skills here and now, while none of the infants are. Non-marrying individuals are often better for sending on dangerous missions, allowing those with close friendships to stay relatively safe at home. Migration is just a quicker way of filling jobs without bearing the overhead of non-working kids.
If it is more like changing the character's hair color or his preferences, that can be "gamed", tweaked as the fort needs. It's not exactly the same as other games where you select the protagonist's preference and the player chooses whether to romance Elliot or Jodi, in that you have little to no say in who takes a liking to whom, if any. That is, unless you are willing to lock them together in the same room until a relationship takes or it does not. Which isn't exactly what I'd call "romance".
I get that the OP has a vision of a game where there are other like-minded individuals. I just don't see how it works in this kind of a simulation without ending up with ham-handed things like bashing your sweetheart over the head and dragging him home by the hair.
Now if preferences were expanded upon, say, if one had a fondness for wren eggs, and chose that from the barrel of biscuits, or the guy who likes apple wood trades out his bed and chair for applewood furniture, great. There's potential for the game itself to accommodate orientations in a PC-friendly manner. Is that anywhere close to happening?