That would depend on how likely same sex marriages are to occur in-fortress. If they were a fifty/fifty proposition, then it probably would be a megaproject to keep them down.
I, personally, have no problem with there being same sex marriage in the game, but I'd rather Toady temporarily fix or optionalize this with the qualifier that he'll implement it again when marriage isn't required to have babies in-game. It isn't in real life, so there's shouldn't necessarily be a requirement in the game.
And as for anyone who talks about practicalities, honor, and duty to have kids, stuff it. Children get subjected to so much Hell from their parents' natural inclinations that there's no good basis in-game for barring same-sex marriages for the sake of child rearing. Need a list?
1. Female soldiers carrying their babies into battle, or even training.
2. Women, knowing full well that snatchers can come at any moment, wandering outside with their babies and no armed escort.
3. Children being allowed to run free, without even the existence of an official crèche zone to protect them. (You can burrow them, but there's no standard for it, so we can assume there's no mandate for us to even bother.)
If women with babies, who had dangerous jobs, were really SUPPOSED to be childbearers, they'd have some instinct to protect their children. They'd hand their precious children off to trusted non-combatants during deadly situations, not carry their babies like damn shields. We all know how common it is, after all. It's why so many of us are careful to avoid female soldiers.