I think some people would find playing the game that way fun and interesting, but I would not. It just adds annoying tedium, at best, or, depending on implementation, if could completely ruin the game for me.
If the only way to play the game is to be "realistic" in regards to pregnancy that could put people like me right off the game.
If a toggle to disable it exists, on the other hand, everyone who doesn't like the idea of disabling prego content can simply choose not to disable it, and can thus engage with the content as it is, but those of us who find it a real turnoff can still enjoy the rest of the game.
It seems pretty clear to me the second option lets everybody win, don't you think?
I agree with you all the other points, though. Especially family related quests; personally I'm all for anything that revolves around trying to manage a 'free use' type situation. I can see how quests to effect the specific rules and duties of the MC in such a scenario could be interesting (effecting how dad and bro see you through quests, so it's the relationship changes that would be the quest rewards), even to the point of getting the MC out of it...or deeper in, to more like a 'family sex slave' sort of thing?