Bachelor Ben Flajnik has a bad-boy streak!

He may have gotten engaged to Courtney Robertson in Switzerland last November, but that didn't stop 29-year-old Flajnik from stepping out on his bride-to-be -- with three different women in one weekend. (See more pics in the latest issue of Us Weekly, on stands now).

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Popping the question on a hill near the Matterhorn Nov. 16, Flajnik found that his feelings toward Robertson changed once he saw his fiancee's abhorrent behavior played out on national TV. And then he turned his attention to other women.

VIDEO: Courtney apologizes to Ben's bachelorettes for her bad behavior

On February 17, Flajnik -- out with a group of four ladies -- was feeling especially frisky, chatting with several women who came up to him at local haunts Monaghan's and the Lion's Pub. When two of them set their sights on Flajnik, he pounced. "Ben seemed pretty toasted, and this woman in red pants came up to him," says a witness. "It looked like an altercation at first, then he kissed her on the mouth and hugged her."

When that woman split off from the group, Flajnik and his friends continued on to a local donut shop with the remaining women -- one of whom had been texting with Flajnik since they met on Valentine's Day, according to a source. After picking up a dozen pastries, Flajnik and the group walked the few blocks back to his apartment.

VIDEO: Courtney flashes her boobs in Panama

"Ben and this girl looked like they were drunk and having a good time," says a witness of Flajnik and his gal, who spent the night at his place and surfaced the next morning wearing the same clothes as the night before. "They were doing weird dance moves, laughing and joking."

VIDEO: Bachelor Ben and Courtney go skinny-dipping

Shockingly, less than 24 hours later, Flajnik was spotted out with yet another woman. While taking his dog, Scotch, for a walk late in the evening, Flajnik and the woman stopped by a waterfront park near the Golden Gate Bridge -- and made out for nearly two minutes!

Says one insider who witnessed Flajnik's kissing: "Ben did not seem to care who saw him, considering his status on a prime-time reality show about finding true love."

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