Mariah Carey -- It's LESBIAN Night!

Just hours after Barack Obama announced his support of gay marriage, Mariah Carey stopped by one of the most famous gay clubs in L.A. -- for some girl-on-girl fun.

Mariah dropped by the Abbey yesterday in West Hollywood -- where a very special Obama-loving lesbian night was in full swing. We're told Mariah stayed for approximately 1.5 hours from 12:30AM til the bar closed.

Sources tell TMZ, Mariah had a great time -- holding court over the sea of X-chromosomes at a table beside the dance floor. Nick Cannon was nowhere in sight.