the two first characters you shattered/awakened can't get pregnant because of something plot that hasn't been fully explained yet - basically they're too closely connected to you? May change in the future. As for molly and some of the others - they're side characters who haven't been expanded on to that extent - yet
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I would like to recommend leaving it as a "post game" bonus. That way you won't have to jump through hoops doing that while also having the myriad other things in the works lol. Then that leaves it open for you to add in fun(ny) quirks for whichever side character, and it won't possibly clash with anything you do later on.