I'm just going to do a future vision guess that the gems can get "pregnant" eventually. My reason for this is that in the main show it is a FACT that gems can get pregnant since rose quartz/ pink diamond did get pregnant from Greg and since Steven is the son of Greg he inherited that trait from him and can get other gems pregnant if the relationship is high enough. Unlike with Rose these gems wont need to give up their gems since Steven is halve gem(diamond)/human so a new gem is formed inside them and they will just experience cravings except for Amethyst who already eats a lot of things. Since the gems bodies are just constructs Steven just needs to cum inside them either oral, anal or in their pussy to have a chance to get them pregnant.