A person might make a display of excessive public affection for another, causing others to remark, “Get a room!” (That is, go to a motel room and show your affection for one another there.) The slang expression “get a room” dates in print to at least 1999.
 
In politics, “get a room” can be used when one is overly praising another in public, such as when a donor or political leader is praising a political candidate, or when a presidential candidate is praising a vice presidential choice.
     
   
Wiktionary: get a room
Interjection
get a room
1.(idiomatic, slang, sometimes humorous) Commanding a couple to stop displaying affection in public, and to rent a hotel or motel room to continue amorous activities in private.
 
Urban Dictionary
get a room
Derisive or humorous comment said to couples engaged in heavy-duty PDA (e.g. swapping spit in the middle of a party) that means your wanton lust is making me uncomfortable (or jealous). The implication is you should get a motel room because you’re practically doing it here.
Get a room you two love birds! We’re trying to watch the game here.
by mandingoe May 24, 2004
   
22 September 1999, Fort Worth (TX) Star-Telegram, “Fall TV The 1999 Season,” Life & Arts, pg. 2:
Example: “Woof, woof, woof woof” is translated as “Get a room” during one of Hunter’s frisky encounters with a date.
     
Straight Dope Message Board
Danielinthewolvesden   
06-03-2000, 04:50 PM
Grace & Dry: (to shamelessly steal another poster’s great line)- Get a room!
 
Walleye Message Central
BE
09-23-2000, 09:05 PM
GEEEEZZ Ness, Get a Room!
 
Google Books
Turd Ferguson & the Sausage Party:
An uncensored guide to college slang
By Ben Applebaum, Derrick Pittman and Burt Falgui
New York, NY: iUniverse
2004
Pg. 25:
Get a Room
An expression used when two people are displaying their mutual affection publicly and it would benefit everyone’s digestive tracts if they would go into a private area before doing the hippity-dippity out in the open. Often yelled across the bar to said couple.
       
Google Books
The Concise New Partridge Dictionary of Slang and Unconventional English
By Eric Partridge, Tom Dalzell and Terry Victor
New York, NY: Routledge
2008
Pg. 286:
get a room! used for discouraging public displays of affection US, 1999
 
Google Books
American Slang
By Barbara Ann Kipfer and Robert L. Chapman
New York, NY: Collins
2008
Pg. 188:
get a room sentence
A request for a public display of affection to be taken to a private location