This person have voluntary sexual relationships with other people, without being emotionally involved. Typically two good friends who have casual sex without a monogamous relationship or any kind of commitment. Or a random people from some Internet club having fun more than once. At the same time, both sex friends may be married to other people and, in fact, engage in sexual infidelity (with their spouse aware of it or not). Still, don't confuse them with lovers, who are involved romantically as well.