Joan Rivers and Ray J In Sex Tape Gag ... That's What She Said!

Ray J has re-entered the world of porn ... and he's gone from blow jobs to nose jobs.

The man famous for making Kim Kardashian a star hit the sheets with Joan Rivers for a very dirty session. Ok ... nowhere near as dirty as he got with Kim, but in old lady terms, it's raunchy.

As Joan does so well ... they faked sex for her WE tv show, "Joan & Melissa: Joan Knows Best?" (which premieres March 29). You gotta watch ... it's pretty funny.

Even though it's all for gags ... Vivid Entertainment honcho Steven Hirsch tells TMZ he did meet with Joan to discuss the possibility of making her his first GILF star. He tells us, "I'd pay her $100,000 right now!"

The question is ...