It will take many iterations to sink in, if it ever does, that it is not him. Rather, it's that whatever it was that made you fall for him, is still active in you.
"Why is it that so many people, when discussing poly for the first time, look only at the issue of having to "share" a partner, rather than at the opportunity for loving and being loved by more than one person?
After all, you fell in love with someone who was compassionate, loving, sensitive and caring... Expecting or asking that partner never to fall in love again, never to love another, is like asking them not to be themselves. I rejoice in the fact that my partner can and does love!"
In point of fact, the "problem" lies with the societally-conditioned assumptions that surround monogamous marriage itself--they are completely asinine and contrary to the way most humans work (I say "most" because there are, in fact, some people who are hard-wired mono. Most people, however, give lip service to mono and stumble around in serial monogamy, infidelity, or repressed frustration).
It's worthwhile to look at the history of marriage itself, and see both how relatively recent the notion of monogamous marriage actually is, as well as how it began (as a property transaction, to assure a landholder of who his heirs were). All of the Prince Charming, fall-in-love-forever crap got added on later.
To me, the traditional (as opposed to monogamy by deliberate choice) monogamous model amounts to a view of relationships that is both based upon and reinforces the very worst personality traits imaginable: insecurity, fear, envy, covetousness, and possessiveness. Personally, I think those traits form a horrible basis for bonding with someone whom you say you love. Love and trust, I think, form a much better foundation for a long-term relationship.
As for whether or not you can help being poly, no, I don't think you can. There is already research that suggests that success at monogamy correlates inversely with the number of copies of a particular vasopressin receptor gene that you have, suggesting that, as with sexual orientation, there is a "relationship orientation" spectrum ranging from hard-wired mono to poly-friendly to hard-wired poly. You can no more help being poly than a gay man or a lesbian can help the way they think and feel, and you'll be just as unhappy forcing yourself into a mono role as any gay or lesbian is when trying to force him- or herself into living straight.
Unfortunately, knowing all of this doesn't make it any easier. You are fortunate that your husband doesn't lash out with hurt and anger, and threaten to leave you. You can, perhaps, ease into this. If there's a local support group, go and talk. Join the livingpolymono list on yahoo. If you do become intimate with someone else, be sure to feed some of that NRE back to your husband.
As for intimacy and sex, there's of course more than one view. To people who love each other, sex can be a physical expression of affection like no other. It can also be a fun romp. :-) To the casual FWB-type person, sex is "just" a fun romp. It can be different things to different people. It might be useful to ask your husband if, as he believes, intimate relationships do not require sex, would he then be willing to give up sex with you?
I'll bet you can guess the answer.
I suspect he is still in the mono mindset that sex is something special between committed partners, and having it with someone else would somehow diminish its specialness when you have it with him. This is, I think, part of the BS package that surrounds the cultural monogamous mindset, but telling him that is unlikely to be persuasive. If either of you have had other partners before your marriage, it might be worth pointing that out, and asking if the knowledge that you had sex with someone else before you met him somehow diminishes the experience with him.