Harry and Ron get caught doing a bit more than dancing with Parvati & Padma Patil at the Yule ball (abrosiis)
Viktor, Cedric, Fleur and Harry all caught in the act while unwinding some stress during the Twi-Wizard Tournament (abrosiis)
I think we all know what the founders had in mind when they made it so that girls could come into the boy's dorms