I can't remember the name of the manga where this happens:
Two girls are dating (young girls, like high schoolers) and have already kissed and made out. They are in a bedroom and Girl A asks Girl B to do oral sex on her. Girl B is freaked out (I'm not sure but it's like she thinks "a vulva is the place from where you pee, so gotta be dirty, right?") but says yes to prove her love for Girl A. She starts doing cunnilingus on Girl A, and feels so icked she gags with a loud noise. Girl A looks heartbroken and hides under the bed covers. Girl B starts desperately apologizing, but then finds the other girl is laughing her ass off (she was using the covers to muffle her guffaws). So no drama in the end.
It's a rather old one, I think. Really can't remember the name.