What sort of examples can you bring up of custom marital contracts that would make people scream in horror? My guess is that people would generally feel queasy about allowing legal enforcement of what looks like slavish or abusive relationships. I think this would be a genuine cause for concern, not because I don't think that people should be able to enter whatever relationships please them in principle, but because in practice I'm concerned about people being coerced into signing contracts harmful to themselves. Not sure where I'd draw the line exactly; this is probably a Hard Problem.

Remember that "enforcing contracts" could mean two things. It could mean that the government steps in and makes the parties do what they said they would - it keeps whipping them until they follow through. It could also mean punishing the parties for damage done on the other end when they breach the contract. For example, in a world in which prostitution is legal, X proposes to pay Y for sex. Y accepts. X hands over the money. Y refuses to have sex with X. The horrific version of this is the government comes in and "enforces" the contract... by holding down Y and, well, yeah. The alternative is the government comes in, sees that Y has taken money from X by fraud, and punishes Y the same way it would punish any other thief. The second option is, I think, both more intuitive and less massively disturbing.