You don't have to be with the mother to be a good father to your child. It's a harder way to parent, but you can still be a good dad.
Just note that if you decide not to be a part of the kids life and she chooses to let someone else father your child that's a decision that you'll have to live with for the rest of your life.
My biological parents divorced when I was a baby. My sperm donor (biological father) decided not to stick around. My mom remarried when I was 5 to a man that I now consider my Dad. I love him, until I get married he'll be the #1 guy in my life. My biological father tried to contact me a while back but he's a stranger and I have no desire to get to know him. I already have a Dad.
Just giving you a possible glimpse into your kids future.