Mary-Kate Olsen and Olivier Sarkozy ‘Very Playful’ at Hamptons Party: Source

Olivier Sarkozy and Mary-Kate Olsen

Michael Blanchard

updated 08/19/2014 AT 12:00 PM ET

originally published 08/19/2014 AT 3:45 PM ET

While Mary-Kate Olsen and fiancé Olivier Sarkozy have managed to keep much of their romance private, the pair stepped out in the Hamptons over the weekend and weren’t too shy to share some PDA.

Olsen, 28, and Sarkozy, 45, hit up the Breast Cancer Research Foundation’s third annual Paddle & Party for Pink benefit in North Haven, New York, Saturday night, which raised $1.5 milliion.

“Olivier was very, very publicly affectionate with Mary-Kate – very playful,” a fellow partygoer tells PEOPLE. “They were kissing [and] touching. They just seemed happy to be together in this beautiful environment.”

At the party – held at designer Lisa Perry and husband Richard’s sprawling bay-side home – Olsen sipped on champagne and stuck close to her fiancé, as well as twin Ashley, pal (and the president of their label, The Row, and company Dualstar) Jill Collage, Molly Sims and husband Scott Stuber.

“They looked like they were happy to be there,” the partygoers adds. “It just seemed that they were very natural together It just kind of works. I know everybody thinks [about] the age difference, but you didn’t get that sense.”

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