Ha! I once (many years ago) wrote a blog post on this very topic.
As someone who has stayed in a couple hundred hostels in a couple dozen countries, I've seen my share of stuff. Basically it comes down to this:
do it where you can without impacting others (this includes not leaving behind any evidence).
Don't be one of the couples who thinks hanging a sheet over the front of their bunk gives them enough privacy to do the deed while others are in the room without everyone knowing because I don't care *how* quiet you are, you're not *that* quiet.
Suggested locations based on other traveler suggestions (because I would certainly never do anything like that!) (these apply to more *ahem* solo activities as well):
* bath/shower rooms after everyone is asleep but only if you can lock the door to keep midnight marauders out and still give them access to necessary facilities
* ditto the above to any common living area but beware that you could be interrupted at any time (I once heard a couple going at it in the living room of a hostel late at night when I was going to my room. Couldn't sit down on any of those couches for the rest of my stay.)
* If you're adult enough to wanna hook up you should be adult enough to plan for it, so get a double room for the night (this is the best, least stressful option)
* Go take a long walk (bring something to sit on)
* Quickie in the bathroom of a bar
* During the day, head off trail a bit into the woods and commune with nature
* Rooftops can be good for some alone time
* Linen/supply closets and laundry rooms can be handy