I wonder if it’s possible to make the switch for the other girls then, with Elizabeth excluded. Though that could make things more complicated.

I say go for pregnancy but build it into the lore of   the game as an active mechanic. The MC could be sterile in the beginning   due to his nature and at some point in the story  he has the chance to do a questline  to heal himself from that   "curse".   If people decide to do that more quests with pregnancy content will open, otherwise they won't. That way you won't lose people who don't want to see it and make more out of it than just an on/off toggle option.