all the gems are non-binary. there is no such thing as a "female body" to them. the wikia even lists them as sex-less so i feel as if most of the community has already discussed this. human gender binary does not apply to them especially since they can shift shape so there is honestly no way you can assign a gender to their bodies.
Technically you can't assign sex, but so far it seems all the Crystal Gems have answered to being idetified as female by others, although this could be just through experience and it could be that most homeworld Gems are Agender, I can't recall any of them responding to or self-referring as female.
Either way, 'acts like a boy' is just a phrasing for "why aren't there any outright masculine/'male' gems?".