See also public display of affection
"Get a room!" is a phrase commonly uttered when a couple, generally at the just-got-together stage, are seen making out or worse in public. But not just innocent canoodling, the whole hog: groping, fondling and even licking.
Not to say that there is anything wrong with these activities but that they would probably be better carried out where the couple can consumate their fresh relationship fully.
"Get a room!" means, with humour, go find yourself a hotel or motel for half an hour and spare the rest of us from your alcohol frenzied pash attack!