Should I get a laywer and establish Joint legal and Physical Custody, or...?

The mother is expecting in 3 months, We dated for a few months and now are just friends. She is currently seeing someone else. I asked her about custody. She would prefer to avoid the courts(she has had a negative experience) and just work with a verbal agreement to share our child equally. This makes me nervous. This is the first child for both of us. She is younger than me. She has been known to make some bad decisions. I'm no saint. I'm wondering if a few years down the road, she falls in love with another person, and wants to move out of state. Can she take my child, and if so, could this verbal agreement backfire and make me wish I would have gotten a laywer. I am leaning on getting one. I just don't want to cause any bad felings by going this route. I do want to protect myself and my child. She told me she doesn't want me to take her child. I would never want to do this unless the child was being harmed. I'm sad that I have to think about such issues before my first child is born. I wish we could be together, but I guess only time and her feelings will tell. Please keep your responses to just the legal aspect of the issue. No personal evaluations of the relationship. I can give you legal backgound, but I don't want to speak negativley of the mother. thankyou.