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^ that doesn't really cover the trope described. The only inversions on that page are mistakes due to lack of knowledge, not "wow we thought you'd be a virgin because of _____ assumption."
So the opposite of Unexpected Virgin?
Might be related to Covert Pervert? Or a subversion of Nerds Are Virgins.
Edited by mightymewtron
^^^Tropes can be inverted in more than one way, and "character you wouldn't expect to be a virgin is" sounds like a valid way to invert it.
That said, this also sounds like it could work on its own.
Unexpected Non-Virgin
Character everyone expects to be a virgin or uninterested in sex (the fat nerd, the homely girl, the guy never known to be dating, the Sour Prude etc.) turns out not to be one (or is outright the most experienced member of the cast).
Usually has an undertone about not judging by appearances or that sex isn't everything, but sometimes used for laughs by Virgin-Shaming a more conventionally attractive character.