My girlfriend is sleeping with my dads new wife

My dad is middle aged (50 something) and he has been married and divorced a lot. He is currently married to a woman who is probably half his age. I am pretty sure he is paying to be married to this woman. Anyway I had a girlfriend and she ended it with me a little over a year ago. I did not really see her for a while besides when she was with her mom. But recently I have been seeing her more and more and I noticed that she was at my dads house a lot. Its possible that my dad was working and she was just there hanging out with my step mom. I dont really know. I did not see them together for a while until I was with my step mom the other day and she was telling me about how she would like to do it with an Asian guy. I thought nothing of it because she has told me about how she likes/want to do it with all kinds of different races. Then I saw her with my ex girlfriend. They were talking about me. They were making fun of how I am a loser and how I would never get a girl again. They were also talking about my dad. They were making fun of how he was so dumb for paying her to marry him. There was a lot of talk about my dad. Then my step mom was talking about how she was going to bang my ex girlfriend. Then she was saying how I was so dumb cause she was just pretending to like me. I dont really know how to feel about this. I dont really know if I should feel betrayed or not.