To be honest, I don't really mind. It does kinda irk me when I get the feeling that the people doing it are only doing so to show off. That's more-or-less high school exclusive, so I don't have to put up with it anymore, although I guess it happens in clubs too (which I don't go to). But it's only a minor passing annoyance.
Hrm, I guess it's also irritating when you're hanging out with friends and the friend(s) are are in (a) relationship(s) are kinda going at it in front of you. I just find it disrespectful and rude, especially if I'm the only one there, leaving me to awkwardly stare at them until they finish.
Otherwise, you know what? There could be an random couple having sex on a table in a bar, and as long as it wasn't my table, I'd just roll my eyes and outside that not really give a damn. Although, if I were to come back to that pub, I'd probably avoid that table.