I want nothing to do with my Pregnant EX?

My EX girlfriend is pissed because i want nothing to do with her. She lied to me about being on birth control. She claims she was on birth control but wasn't on it long enough for it to be effective. I was going to break up with her when she told me she was pregnant. I stayed with her for awhile until i decided it doesn't matter if she pregnant with my baby i don't love her. I love my best friend Leslie and we have finally started dating. Leslie supports me she knows i was lied to and she told me it really isn't mine because i don't love her mother. I told her i want nothing to do with her or the baby once it is born. I will however pay child support. My parents and friend have told me it's i don't have to be in a relationship with her but not to take out my anger towards her on my baby. That said i will regret it not being there for important moments. I feel justified in my stance. Why can't she accept reality?