While I don't mind it on the non sharing save Kathryn is one of the few LI I wouldn't mind knocking up I rather it not be forced either nothing is worst than taking choices away even if it would make a bit more sense in the narrative. But I'm not sure if I would want Kathryn to get pregnancy on my sharing save if it's only comes in the form of a ending. But if it adds some branching / drama or issues for Luke and Kathryn to solve before the end of the game. Then I wouldn't mind her getting pregnant by another man. So yeah would have to wait and see what I would do on that save when it does happenTrue, it definitely needs to be on the pure/sharing path, but maybe it has to be "forced" on that path because, as you've said, Kathryn already expressed a desire for kids to Luke on that path, having it as optional would be weirder in fact.
But it needs to be optional on all the other paths, as well as multiple possible paternities (but not too many, one specifically for Alex, one specifically for Luke's dad, and make a generic pregnancy ending if Kathryn got impregnated by any other random encounter).
If Bobby can do all of the above, this game will easily be 10/10 for me.