TL/DR: An ex-girlfriend wants me to father a child that she will raise as a single mother without me involved. We still hook up occasionally so she wants to conceive naturally. I have no problem with this except that I don't want any financial responsibilities. Is this a bad idea?
Long version: An old ex-girlfriend and I dated for about six months and we both realized that we were not compatible outside of the bedroom. We have continued to hook up 2-8 times a year for the last four years. I have only seen her when we have hooked up and it has always been cool - no awkwardness. We are friendly and text a lot and talk about other people we are dating. Last week she called me which is weird because we never talk on the phone. She told me that she really wanted to have a baby and she has not met anyone she wants to raise a child with. She asked if I would consider fathering a child that she would raise completely on her own without my involvement. I have no problem with that and would not feel any ownership over the child so I said yes. Then I asked if she needed me to go to a sperm bank or something. She said that it would be cheaper and more fun to conceive naturally. Again, I have no problem with that. My only concern is about what would happen if she was playing me and suddenly demanded child support. If I have her sign something saying that any child we conceive together would be 100% her financial responsibility, would that hold up in court? Are there other issues I might not be thinking of?