A hot girl's vagina.
Very selective on who's dick is permitted through the membrane (her pussy lips.)
Will often deny nervous, non-jock types. The membrane judges largely on physical appearance, not on the pretense of a good personality or wit.
Guy 1- So did you sleep with Elena in the end?
Guy 2- No, she had a selectively permeable membrane.
Guy 1- ... So she turned you down?
Guy 2- Yeah.
Guy 1- Well just say that then!
Guy 2- I needed to make an example for a definition for urban dictionary, okay?!
Guy 1- Oh, right... Wait a minute, am I on TV?!