I personally couldn't be in a relationship where I was asked to hide a major part of who I am just because it makes the other person uncomfortable.
He also needs to realize that it's contradictory to say "I don't mind if you ARE poly, but I don't want you to ACT poly." Either he accepts it or he doesn't. It's like those parents who say "I don't mind if my son is gay, as long as he marries a nice girl one day." Translation: "I don't accept what you are."
The other ingredient is that sex can lead to emotions. He accepts that you have sex with other people, but a natural consequence of that behaviour may be that you fall in love with someone as a result.
However, he has given you that go-ahead, so it may work out to stay with that for now, and just give him time for the poly-idea to gel in his head. Meanwhile, you can have your sexual encounters. Then if you fall for someone, you can deal with that as a real situation, not some "maybe some day in the future" scary possibility.
What it comes down to is whether you really are willing to give up your nature just for him to avoid icky feelings. Sure, polyamory is not what he signed up for, but things change. If he's not even willing to entertain the idea enough to learn about it, is it fair for him to say that he loves and respects you as an individual?
I recommend that you and he read Tristan Taormino's Opening Up
. It will give both of you a better understanding of the different forms of non-monogamy, as well as some strategies to apply in your own life.