Trouble accepting girlfriends past

I have a wonderful girlfriend. We've been together for almost six months now, and our relationship works well on all levels. But there's something that keeps bugging me, something I just can't get over, and that is her past. She's only had one partner beside me, but beside the fact that I don't get to be her first, the nature of the sex she had was both unplanned, spontaneous and edgy on so many levels. She's been completely open to me about what happened, so I know most of the details. A mistake, maybe, since I can't stop thinking about them. Basically she's grown up in an island community, and her and a friend had just been swimming and were on their way home. This older man, a guy in his 40s saw them walking from his house, and invited them in for some lemonade. They were like why not, and accepted the invitation. So he took them indoors. They knew him from the local community, and he has a wife and two small children. That didn't prevent him from flirting wildly with them. He told them they were very beautiful, that their future boyfriends would be very lucky, and asked them about their sex-lives, all the while they were sipping his lemonade in his kitchen. When he got to know they were completely inexperienced and embarrassed because of it, he offered to help them out with their problem. They asked him if he was serious, and he said yes. His wife was away, he explained that they had an open relationship, that it would be fun and that nobody needed to know. After some reluctance they said yes, they wanted to have sex to get it over with. Upon having their consent, he escorted them to the kitchen table, and told them they needed to use it in order for him to service them both at the same time. It wasn't exactly what they imagined, especially the position involved. From now on things happened really fast. When he had got their bikini bottom off, he more or less lost his cool and self control, and was in a real hurry to undress and get them in position. For them it was uncomfortable and painful, especially since they were so unprepared. It wasn't without comedy, because upon achieving penetration with both girls, he lost his balance and almost made them fall. He had three intercourses with them this way, two lasting only a few minutes, and one taking like forever. He didn't use a condom. When they were done, he wiped away the mess, and they could go to the bathroom. They were bleeding a little all the time, and he suggested they could use his wife's tampoons, which were much too big. But they had no choice if they wanted to hide what happened. He gave them each a hug, and that was more or less it. They had to make an effort not to walk weird on the way home, and got the pill soon after. Now, I realize the sex was as random and impersonal as can be, but I still can't get over it. They of course regret it now, but didn't really think it a big deal at the time. I'm so angry he got to have them this way, so easily and so crudely. How can I get over this?

Hi Alestar..... I always wonder how old the poster is that I am responding to? Im assuming you are 20's or younger?
Anyway, you said she is wonderful and the relationship works well on all levels..... I believe that, as she must have felt total trust in you to have told you all that in the first place.
So accept it as just something you wish she hadn't told you, and move on. Don't dwell on it, because it has nothing to do with you whatsoever. Nothing... not your business in any way.... just a part of her young past.
Instead, be grateful for the wonderful relationship and the trust and openness she gives to you.
Find a way to forget it and move on.