If that's the case, as time goes on, Stevonnie would also be getting more feminine. What wins out?
Ultimately, if you claim it "Makes sense," you have to say, "because gem space magic." Which could be the same explanation for why she _doesn't_ grow facial hair.
If it's supposed to be like a "realistic" hermaphrodite, humans never are and never have been. If it's not supposed to be realistic, then realism doesn't need to apply and you can make it anything you want, even largely feminine with few or no masculine traits.