I feel like the bit that's sort of being glossed over/missed is that the bf in the relationship is making his issue (my gf has friends that want to fuck her) into his gf's issue by introducing the boundary of "you're not allowed to have friends that want to fuck you". That should be an unreasonable boundary for anyone (barring edge case scenarios that involve informed consent between adults) because one person is taking their internal issues and externalizing it on someone else (presumably) without consent.

And then the gf flips that wrongheadedness back onto her bf by saying "if I'm not allowed to have friends that want to fuck me, then you're not allowed to have friends that want to fuck me either". It's a humorous response that illustrates the hypocrisy of the first boundary introduced by the bf, and also hints at the slippery slope nature of forbidding relationships based on uncontrollable, external criteria like "does someone want to fuck you".