There is a strain of libertarian contract theory that (while perhaps still problematic on feminist grounds in its likely implications) is more compatible with the idea of enthusiastic consent.
Rothbard's position was that the only proper subject of an enforceable contract was the transfer of title to alienable property. Therefore, the only permissible use of force to compel specific performance would be to compel title transfer. A slavery contract, under this theory, is unenforceable because one's self and one's labor are not alienable.
This breaks down around edge cases relating to alienable property strictly necessary for life, though there's a case to be made that any property strictly necessary for life isn't alienable. That's a hole big enough to drive a system of socialized medicine through.
Rothbard's position was that the only proper subject of an enforceable contract was the transfer of title to alienable property. Therefore, the only permissible use of force to compel specific performance would be to compel title transfer. A slavery contract, under this theory, is unenforceable because one's self and one's labor are not alienable.
This breaks down around edge cases relating to alienable property strictly necessary for life, though there's a case to be made that any property strictly necessary for life isn't alienable. That's a hole big enough to drive a system of socialized medicine through.