The 43-year-old Lost star makes the admission in the May edition of Playboy magazine, and is happy to relive the monumentous occasion. He explains his lover was two years older than him and more experienced, but even that couldn`t stop the event being painfully embarrassing.
"I was 12, it wasn`t her first time," he tells the publication. "I can actually see the event in my mind`s eye, like photographs. It was in Dubois, Wyoming, where the population sign probably says, to this day, about 1,000.
"It happened literally on the ground by a river while a rodeo was going on in town. It was absolutely terrible and awkward - just two f**king kids lying down and pulling our pants down."
Matthew`s sex life eventually perked up, with the actor revealing all about his exploits while at university. The Hollywood hunk attended Columbia University where he was a star in the football team, which meant he was also a big hit with the ladies.
"My sex escapades at Columbia were in my freshman year," he confesses. "That`s what you`re supposed to do. We didn`t get any of the benefits of being football heroes. There was plenty of sex but no football-hero sex."
Matthew is now happily married to Margherita, who he was two kids with. The pair wed 19 years ago, and are still very much in love. Matthew can`t imagine being without her, explaining they have a very happy relationship. "She`s the love of my life. I am a man, and I am an appreciator of women. At the same time, Margherita is the sh*t. I can honestly say we`re good together," he says.