50 Cent's Response in Revenge Porn Case, It Was Just a Repost

50 Cent's going with the ole "everyone's doing it, Mom" explanation for why he posted a screengrab from 'Love and Hip Hop' star Teairra Mari's sex tape.

Fiddy filed his response to Teairra's revenge porn lawsuit, and in the docs he says the image he posted -- showing Teairra with ejaculate on her face -- was already all over the Internet when he reposted it. He says the fact others beat him to the punch protects him legally.

As for Teairra's argument he went beyond a basic repost by applying a black & white filter "to highlight the color contrast of the semen" ... 50 says even if that's true, he didn't change the nature of the photo.

50 also echoes a statement the judge made last month -- that the photo doesn't fall under revenge porn because it doesn't show genitals or a sexual act in progress. In flagrante delicto, in Latin.