Because if Super Girl really wants something, she will find a way. Because that’s what makes her Super. The hymen thing is easy: it can be pierced with a penis belonging to a person with the same superpowers. Everyone else’s is simply broken off in a fit of superpassion.
Either she: A) Visited a mall in the bottle city of Kandor (when she was miniaturized after being exposed to an odd variant of red kryptonite) and had it done there or B) has some of those magnetic earrings.