Hmm, why set it up like that, though? If you're expecting him to fail, you're putting yourself in a position to be the victim and him in a position to feel like an asshole. Resentment on the one hand and guilt on the other, not fun. Not only that, if you *really* believe that he'll screw up and cheat eventually... what if he never tells you, what if he just keeps it a secret because he thinks that would hurt your feelings less? Then there's actually a safety issue, because you can't make informed decisions about how much risk you're comfortable with. Wouldn't it be better for everyone if you just said, straight up, "I don't mind if you hook up with a random guy, as long as it's not more than once a month," or whatever you think you'd be comfortable with?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.