Random idea / theory SOMEWHAT NSFW
Note this is somewhat NSFW:
So Rose and Greg most likely had sex and produced Steven, but than it had me thinking, how does Rose have the ability to be able to reproduce. Since gems most definitely dont have human anatomy, well not ALL of human anatomy but you get my point.
A new head canon I have is that over centuries of being in love with the human species, either through reading books on anatomy, or another idea is that whenever a human died in some gory way, like say, a lion attack, (I did that on purpose) she studied their biology to be able to replicate it for her own form.
How she changed her form is maybe she poofed herself and redesigned her inner body (if they fully control it im actually not fully sure) and gave herself accurate human anatomy and maybe took eggs from a killed / slain female human.
If you have any thoughts or something that can add to this, just reply, hope you enjoyed my theory!