It sounds like this is a non-negotiable point for you. That's perfectly acceptable.
He needs to know that you are not willing to be in a non-monogamous relationship. Not now, not ever. If he wants non-monogamous relationships, he should put that down on the table before allowing someone to fall in love with him inside of a monogamous relationship.
Without knowing him personally, I can't say whether or not you should trust him. Does he have a habit of saying one thing and doing another? Has he done anything yet to break your trust? Do you know if he's cheated on women before?
One thing to note, his desire to sleep with other women is not going away, any more than your desire to be monogamous is going away. If he's adamant that he needs non-monogamy to be happy, then I don't see it working out.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."