For a guy who spent years being trailed by a camera crew from MTV, Nick Lachey still appears to have a thing or two to learn about keeping his private life -- and private parts -- private.

TMZ.com reports the square-jawed crooner, 33, and girlfriend Vanessa Minnillo, 26, are trying to put the kibosh on photos showing them in flagrante delicto -- and how -- while soaking in a hot tub during a recent getaway to Puerto Vallarta, Mexico.

A source says the salacious snaps are "Paris sex tape-level scandalous," which we assume means Nick and 'Nessa look really, really bored as they get it on. Another indelicate detail that takes us to a scary visual place: The flexible flames were apparently captured in a variety of positions.

The risqué revelation comes on the heels of Lachey and Minnillo's skin-tastic appearance in the latest issue of Life & Style, which features censored images of the swimsuit- and tan line-eschewing twosome de-toweling before climbing into the Jacuzzi for a smooch session.

"His family is not going to be happy," a pal told the magazine of the au naturel shots. "This isn't the kind of stuff they want to see."

(Hey, we're right there with 'em -- and doubly so for the far more intimate images.)

TMZ says Nick and Vanessa, who recently took heat for photos showing her engaged in some knife play with Lindsay Lohan, have sicced pitbull attorney Marty Singer on would-be photo revealers.

The lawyer, who calls the paparazzo's actions "outrageous and malicious," has warned the tabloids that a lawsuit will ensue if pictures of the lovebirds' alleged lovemaking are published.

"The photos that were taken of our clients by a Mexican photographer violated Mexican law and were a clear invasion of privacy," he rails to TMZ, "and we have threatened to take legal action against third parties who publish the photos."

Lachey, for his part, is taking the erotic exposure in stride.

"Where's the scandal?" he maintains to OK! magazine. "I was in Mexico with my girlfriend of a year, celebrating our anniversary on a private vacation. It's not like I was caught with a Mexican hooker. We've all gone out and had a few too many and done something stupid. We've all made mistakes."

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