@tylo You could always first map the yes/no into integers, associating 0 to 'yes' and a random number to 'no' before applying the SMP so that with high probability no information is leaked if both wish to answer 'no'.

You could change the protocol to have an authenticated server, for instance by securing communication between a client and the server with TLS. But I don't really see why anyone would want to ensure the server is the one generating the secret shared key for the clients. Using an usual key exchange seems so much straightforward.