There is no evidence there was sex or no sex. But one thing for sure is, if the Founders of Hogwarts does not want the kids to have sex, there will be enchantments in place to prevent that. such as the biysboys not being able to enter the girls dormitory.
It is unlikely that the kids were able to have sex in Hogwarts since enchantments in the walls could easily prevent this, if the founders frowned upon it. Why was Harry able to break so many rules? Most of his actions were normal just at the wrong time (in the dark of night).
They could get lucky at Hogsmeade though.