My dad is a horny, immoral dog!? help :(?

I can't even look at my dad in the face anymore....he and my mom got divorced 2 years ago and it was sad...but I got over it eventually. I knew it was for the better to have them split up then constantly fight. My mom has started dating and I am happy for her (very decent men). My dad on the other hand is seeing a 22 year old!! he's 50...I AM 21..he is seeing a girl that is the same age as me. He wanted me to meet her for the longest time (they have been dating for 3 months...I have been dreading it all along). I finally gave in and decided to let him introduce her to me for my dad's sake. The girl is absolutely breath taking! I feel so intimidated by her...she is so smart, read so many books, travelled the world, is from a good family...I mean, no offense to my dad or anything but what does she see in him? She has a bomb shell body (she's around 5'8)...long legs, flat stomach, tanned, beautiful face...as if her features were drawn on. She has a small nose, full lips, pearly white teeth, arched eyebrows, green eyes, olive skin. Anyway, she was very sweet to me and made such a nice dinner when I came over (and I can't even cook lol...i felt like such a loser). My dad kept on her when he thought I couldn't see and I just felt so damn weird. I felt like I was a child yet she was my age...I didn't like it at all...She invited me to a movie next weekand (just me and her) to get to know each other but I really don't want to be friends with her. a) I know myself and this is going to be constant competition b) I don't want to encourage her going out with my dad! c) I don't need conflicts with my mom. It's like my dad isn't my dad anymore but just a horny aquaintance...i feel like crying but on the other hand am so angry with him. I get even more ticked off when this girl tries to suck up to me...am I doing the right thing by not encouraging it? or do you think i did the wrong thing? please help :(