Lay Down the Law
Because, really, your boyfriend is not a human being. He's one part child and one part tiger catnip. You can't tell what you're worried about, and you certainly can't trust him. You must connive. Hint that you will cheat also, lay out metaphorical situations that you pretend apply to you but that obviously apply to him, and then give an ultimatum that doesn't actually relate to the problem (don't cheat on me), but that you imagine might cause the problem (company softball games) — in the terrifying feline parallel universe of your mind.
What happens if we cross the line to Men's Health, and try to find some insight into why a man might cheat on his obviously awesome, obviously not a tigress, girlfriend? Why, turn the page, dear reader.