'American Idol' Mansion -- Former Orgy Porn Den

The "American Idol" kids are following in the footsteps of orgy-lovin, sex-crazed, swingers -- TMZ has learned -- because the mansion they just moved in to was previously used for PORN!

Sources tell TMZ, the new digs currently housing the top 9 finalists on ‘Idol’ is the same place Playboy TV shot season 2 of its racy sex show "Swing."

For those you who haven’t seen it ... "Swing" is a show about couples who move into a house with the sole purpose of “living out their erotic curiosities” through swinging ... i.e one big fat excuse for banging each other as often as possible.

As for the specs -- the former sex den is a 10-bdrm, 16-bath, 16,000 sq. ft swingers dream ... now housing the squeaky clean -- soon to be Disney World attraction -- "Idol" finalists.