Jesse Grant / WireImage 1 / 8
Jesse Grant / WireImage 1 / 8

Ashton Kutcher and Demi Moore are putting up a united front -- and considering legal action -- over allegations that he cheated with a "hot young blonde." The current issue of Star claims the Twitter-happy couple's marriage is "in danger of falling apart" because the actor was supposedly spied "kissing and groping" someone other than his missus.

RELATED VIDEO: Demi shakes it

According to the tab, the ostensible handsiness took place by an "out of the way" bathroom (oooh, romantic) at see-and-be-seen Los Angeles eatery Madeo, which seems like a strange rendezvous choice for a high-profile star with an even higher-profile spouse.

"Ashton had this gorgeous girl pinned against the wall, and he was totally making out with her," contends a spy. "I was shocked to see him sucking face with a girl who obviously wasn't Demi."

RELATED VIDEO: A "true celebrity" bathroom encounter with Demi

The detail-oriented source also maintains that the actor was wearing his baseball cap backwards "for easier access to the thin and leggy girl's lips."

Not so, retorts Kutcher, 32, who will celebrate his fifth anniversary with Demi, 47, later this month.

"I think Star magazine calling me a 'cheater' qualifies as defamation of character," he tweeted Wednesday morning to his 5.6 million followers. "I hope my lawyer agrees."

Ashton's ridiculously well-preserved missus responded with an enthusiastic, "No question!"

That's not quite the reaction that Star was expecting her to have over the so-called "steamy session": "When she finds out that it happened," a "friend" tells the tab, "she'll be devastated."

Moore also backed up her hubby when he opined, "STAR magazine - you don't get to stand behind 'freedom of the press' when you are writing fiction."

"Excellent point my love!" she replied.

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If Dashmi don't seem worried by the tabloid claims, perhaps it's because they recently weathered something far more serious -- something that would have undone a less secure couple.

Back in July, they detoxed together on the Master Cleanse diet, which involves drinking a concoction of maple syrup, lemon water and cayenne pepper. If that couldn't tear 'em apart, then this probably won't, either.

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