Got involved in my parents sex life

My parents are of the hippie, new age kind of type. They are very sexually liberal, whereas I tend to be more conservative. From time to time 'Ive been bothered by them having sex in the living room, and they walking naked through the house. I have a beautiful girlfriend, and recently I decided to invite her to my home. We have been together for six months, and my parents invited us for dinner and wine. My parents asked alot of personal questions and soon enough found out we hadn't slept together, but that my girlfriend had started with the pill in anticipation for it to happen.They made the outrageous suggestion my father could take my gfs virginity in order to make our first time together easier. She is 18 so technically it would be legal.

They went on and on about it, and claimed this used to be quite normal in historical times, the father helping the son by taking the virginity of his bride. Through the wine and constant insisting and nagging, they managed to pressure us into saying yes. A couch was converted into a bed and my gf made to lay down on it. My mother and I sat behind them, and she soothed me while my father went through with two intercourses with my gf. It happened in a normal or standard way I guess, but he used no condom. For the most part, all I could see was his butt. Afterwards my parents congratulated each other with a job well done. I was more or less numb or in shock. My girlfriend had been passive. She'd felt some vaginal pain at first, then tried to get used to what was happening with her. The table and the mess was cleaned, and after giving us some assurances and making some jokes between themselves, they went two bed. Only now did I realize what had taken place.

Lather that night, I felt the need to reclaim her, and asked her if I could sleep with her. She said yes. It felt good and was intimate, but when I went down on her, I found a black hair that certainly didn't belong, and she smelled a bit fishy. Basically I lost my virginity in my fathers sloppy seconds. The day after my parents pretended as if nothing happened, but through some inquisition they managed to find out we had been intimate. They claimed it had been easier with their "help" and I was assured she was all mine.

As some time has passed, I'm feeling increasingly resentfull. I wonder what their motivation could have been? Sexual pleasure for my father, or did they want to humiliate us? When it was going on, my father looked back at us and asked if we liked the view. My girlfriend of course regrets it, and says we should try to fight through it and get over it eventually. My parents stepped far, far over the line, ruined a special moment for us and made a dirty mess of our love life. Thanks to my father, I'll never sleep with a virgin, as I want to keep my girl. I want to punish them somehow, but I don't want to go as far as to disown my parents. Should I demand an apology from them to both of us? In that case, how can that help us? Or should I simply try to live with it? They of course have noticed my anger.