Don't change clothes now, girl. You want to use one of the two acknowledged methods of proving to his parents that its a done deal between you two (the other one is showing up pregnant).
@FiddlePop Well, if their underwear is soaked, they can always go commando. I've had to do that a few times when wearing my friend's clothes because mine got something spilled on them or got soaked due to rain or the sprinkler.