I would not. I'm loyal to one person, if I say someone is my life-mate then she's the only one I care about. Oh, would I look at another person? yes. Have sex? no, no matter how beautiful said person was I commited to someone who gave me her trust and as such I couldn't do it nor do I want to.
Oh, if my mate knew and we had some sort of "open relationship"? Then I'm not sure but even then I'm going for no to be honest.