The "National Enquirer" claims a LOT of sex happens behind-the-scenes at "Dancing with the Stars" . . . and they're blowing the lid off of it.

--A so-called "production source" says, quote, "The number of affairs has gotten way out of hand.  There's been so much casual sex behind the scenes that you'd think it's a swingers club, not a dance show."

--They posted a rundown of some of the alleged hook-ups, including:

--Derek Hough:  Shannon Elizabeth and Maria Menounos

--Julianne Hough:  Helio Castroneves and Chuck Wicks

--Mark Ballas:  Shawn Johnson and Sabrina Bryan

--Maksim Chmerkovskiy:  Karina Smirnoff, Willa Ford, Erin Andrews and Peta Murgatroyd

--Cheryl Burke:  Chad "Ochocinco" Johnson, Drew Lachey and Rob Kardashian

(--You can find the full "report" at NationalEnquirer.com.)