The `Two and a Half Men` star - whose marriage to Demi Moore has been rocked by infidelity claims - was joined naked in the hot tub of his San Diego hotel room by 22-year-old Sara Leal and her friend Marta Borzuchowski and after Marta left, Sara claims they had sex twice in the early hours of September 24.
She told The Sun newspaper: "We had sex twice. Everything felt natural. We were having a good time. One thing led to another. He was good but it wasn`t weird or perverted or creepy. I was attracted to him and he was attracted to me. He had great endurance. We were up for a while. It was about two hours."
Sara met Ashton after joining some friends at a party in the suite next door to the actor`s in the Hard Rock Hotel. Most people eventually left, leaving the former `Punk`d star`, Sara and some of her friends alone drinking shots.
Ashton then invited them to his hot tub - and told the group he was "separated" from Demi.
Sara explained: "I wasn`t wearing anything. We took our clothes off right before we got in.
"In the hot tub it was me and my friend Marta and Ashton and his friend. Later my other girlfriend, Alexis, joined us but she just put her feet in and didn`t take her clothes off. Ashton looked hot naked.
"Alexis started asking all these random questions. She was like, `Aren`t you married?` He said, `I`m separated.`
"That`s when it first occurred to me whether he was married. I don`t really keep up with celebrity couples ... If I had known from the beginning he was happily married, none of this would ever have happened."
Ashton and Demi - who are currently on a Kabbalah camping retreat - have not commented on the speculation about their marriage, except for a few cryptic messages on twitter.
Ashton has written: "When you ASSUME to know that which you know nothing of you make an ASS out of U and ME. (sic)"
Demi took to her page last month to post: "When we are offended at any man`s fault, turn to yourself & study your own failings. Then you will forget your anger. Epictetus (sic)"