What's wrong with a consensual slave contract between two adults, if?
If you have a level headed sub who's not afraid to tell you when things aren't okay, it's just a part of a D/s relationship. If you aren't expecting it to be legally binding, and the sub knows s/he can walk away at any time, then there's nothing wrong with it at all. It can go wrong when the sub starts feeling like they really have no choice in matters since they signed a contract. Like everything D/s, be rational about it. These things can be lots of fun, or things can go horribly wrong. Make sure the two of you are looking out for eachother's best interests and you have really good communication. While those two things are essential with vanilla relationships, they are absolutely, without question, a necesity in D/s.