I'm not convinced by your arguments. Yes - men sometimes cheat on their girlfriends, yes - an open relationship brings its rewards (like, sex with whomever you want whenever you want). But by attempting to fuse these two arguments into this: 'Men are cheats by nature so we may as well legitimise my boyfriend's intention to sleep around,' I think you're making your TRUE feelings on the matter quite clear. And these are: You think it sucks that men can't keep their snakes in their trousers, but you feel powerless to do anything about it. And this is where I think you're wrong!
The urge to 'spread love' may be stronger in men than women but there are plenty of blokes out there who are interested in pursuing a loving, trusting, one-woman relationship. They may like to LOOK at other gals, but they don't necessarily feel inclined to touch. It seems to me that this sort of man would be more suited to you. I don't think you're exactly CRAZY for considering an open-relationship (it's good to face sex and relationships with an adventurous mind) but you're probably deluding yourself in thinking this lifestyle would suit you, as you don't seem to share this guy's interest in sleeping around.
I can picture scenes of great sadness, with him prowling around town and you sitting at home alone wondering what mischief he's getting up to tonight. I'm guessing jealousy, anxiety and disappointment would creep into the arrangement before long and this would quickly morph into bitterness and anger. In time, you wouldn't be able to face your lover without suppressing a strong desire to slap him across the chops and beg him to see YOU as the woman who makes monogamy worthwhile. Which of course he wouldn't do! Needles to say, this is all wild speculation on my behalf, and yet; I simply can't suppress this feeling that a relationship with this guy will go horribly wrong. Please reconsider.