Mariah Carey supports gay marriage! The sexy songstress partied until 2am at West Hollywood's The Abbey Food and Bar ... an LA gay bar also loved by Britney Spears.

The mama of twins left hubby Nick Cannon at home to celebrate President Obama's support of same-sex marriage.

A source from inside the club tells People:

    "There was no formal announcement that she arrived at the club, but she and her friend B. Scott (pictured above) held court at one of the main dance floor tables – so she was very visible. She was very nice and gracious."

The source adds that the DJ played Mariah tunes which caused the crowd to "go crazy."

Nick's wifey even took some shots of Beluga vodka. You go, girl!