Floyd Mayweather -- I Support Gay Marriage ... Unlike Manny Pacquiao

Floyd Mayweather is squaring off against Manny Pacquiao ... and once again it's NOT in the ring -- this time they're fighting over gay marriage.

With the media still buzzing over Manny's controversial condemnation of gay marriage ... Pretty Boy Floyd released a counter-statement on Twitter ... saying, "I stand behind President Obama & support gay marriage."

He added, "I'm an American citizen & I believe people should live their life the way they want."

These two can't agree on anything ... so we gotta ask ...