I just realized, in the time between Greg meeting Rose, and Steven's conception, Rose must've been poofed. There's slim to no chance she already had a "receptive female organ" before they met, and it's been established that the two never fused, so she most likely had to undergo some change that allowed her to conceive. Yes, gems can undergo transformations, but they drain their energy. I don't think that even Rose could hold a transformation for the (approximately I'm guessing cause I dunno about gem gestation) 9 months it would've taken to "grow" Steven. Even if she could, I doubt she would risk it. Makes me wonder what poofed her. I bet it will be some thing where she was defending Greg from attack, then she got hit, and he whisked her gem to safety, perhaps himself being saved by Pearl or somebody at the last moment. Regardless, she most likely got poofed, and one way she decided to change herself was to make Steven possible.