James Deen -- Farrah Abraham Concocted Sex Tape Plot

Here's a shocker ... "Teen Mom" Farrah Abraham's so-called "sex tape" with porn star James Deen was a SETUP ... devised by Farrah herself ... so says her XXX co-star James Deen.

Deen tells TMZ, he was approached by an unidentified porn company to shoot the porno with Farrah over the weekend ... and they did. He says the plan was to pass the porn off as a "sex tape," but the plan fell apart.

Deen believes the ruse began to unravel Sunday when he and Farrah were shot leaving the Vivid Entertainment offices, holding hands.

He says, "I think what happened is that [the porn company was] going to pass it off as a 'sex tape' and somebody saw us coming out of a building together and people asked me what's going on ... so I was like we're making a porno!" (Deen wouldn't say if the "company" was in fact Vivid.)

And the kicker ... despite Farrah's poorly-worded protests, Deen says she was the one behind it the whole time, telling us, "Everybody's trying to make it this story, but really [Farrah] wanted to make a sex tape."

But how good was she really? We had to ask ... and Deen doesn't hold back.

We should say ... it's possible a legitimate Farrah sex tape exists somewhere ... but according to Deen, this ain't it.