DO head to the bathroom. This is a great place to do the nasty. You can lock the door and the shower makes for easy clean up after. Well, only a little bit because you might get a little dirty while in there but hey, comes right off!
DO head to the roof. Tons of hostels have roofs and tons of them keep them unlocked. No one one is going to bother you (unless they get the same idea- though maybe “the more the merrier” is a philosophy you take with you all the time?). Bump uglies under the stars all night long. It will be the most romantic time you get in a hostel.
DON’T have sex on the top bunk. You’ll shake the bed. It will creak. You’ll wake up everyone, especially your bunk mate. It’s just mean- there are other places to go. Don’t be that guy or girl! (or any combination of the two!)