We had a hard time with this and sometimes when I'm thinking about the finances it comes back up.
Dp and I have been together for 6years, celebrating our 4th wedding anniversary tomorrow.

We met when DS was maybe a month old.
I had a very, very short relationship with DS' father and ended up with a great parting gift!! We have not seen his father since a few weeks after we found out I was preg. Even though I haven't seen him, he has called a few times and usually they are not good phone calls.
I know for DP it bothers her that their is another person out there who could claim her son. She has been there for everything and I understand her position and why she didn't want to have to worry about it for the next one. Someone tried to tell me once we made our decision that, it wasn't "fair for the child to have a vial for a daddy" (her words). All I could say was how fair is it for my DS to grow up knowing somewhere out there is a man who for some reason can't anti up and be a real man and be his own kids father. Let me stop me ranting....
I know there is paperwork that you can write up, but it still doesn't mean that it will stand up in a court. Each state is different and even though alot of states are becoming more "family" friendly there are still those like the one I'm in who don't get it. It's a tough call, I've heard of people making it work. I have a friend who's doing it now and used a friend as KD, they're co-parenting. Good Luck what ever you choose, Nae