Alright, let's add my waffly contribution... haha...
Your husband is just scared that after finally achieving wow status, the addition of more 'wow' people is going to make things ho-hum between you again.
As someone early in the posts said - it's important at this stage that you do put things in place to make sure that you don't forget each other. I'll be completely honest - my GF neglects her husband. I've said this to her before. They have some great moments, but she could do more. They don't really go on dates, she spends a lot of her evening time talking to sub boys online. They've been married for over 11 years and have had many problems. Ho hum territory and hell territory. Poly has really helped their relationship, compared to what it was. But she really could make more time for him. If she did, she'd probably find that he was less insecure and less jealous... i.e. everyone's happier! So I hope you two can still make time for each other.
Anyway, you sound like you've got your head screwed on... so hopefully you've already thought of that!
My next question is, what kind of poly do you think suits you? Polyamorous, where you are open to loving others? Polysexual, where you only want to have extra sexual partners? Do you believe in husband as your primary and others as secondaries? Would you ever consider three or four of you living together? These are useful things to think about and to see where your husband stands on it.
Onto the coolness factor!
I am in agreement with the other posts here.
1. There are no guarantees that you will stay together, even in monogamy
2. Cheating happens because we want others. If we leave the old person for the new person, it's often because we think we have to choose
3. Promoting each others freedom can equal more respect, which can equal stronger bond with each other, more appreciation, more love, more happiness
4. In poly, the concept is that you don't have to leave, because you can have plural! The new person is exciting - but exciting doesn't mean stronger, better, more important.
Maybe some personal experiences will help your husband?
I can tell you some of mine, if you're dying for more babble.
a) When I have a date with, or sleep with, someone else, the first thing I want to do afterwards is hear my girlfriend's voice on the phone, be with her, hold her, even have sex with her. My desire, commitment, love, energy, doesn't fade because of others. It actually increases. We both report these strong feelings of overwhelming love for each other after being with other partners. It's amazing. I value her so much.
b) I was in a mono/poly relationship a few years ago. 5 years of mono with my boyfriend. I adored him. But I met a girl that I had an extreme sexual attraction to. He ended up giving me the green light and I dated both for about 6 months. If I could have had him and her, I would have stayed with both. I loved him in a way that I didn't love her. I wanted her in a way that I didn't want him. I was so torn. But, I had to pick one, so I picked her. Yes, because she was new, because I didn't foresee the hellish relationship to come (haha). BUT if I could have had both, I would!
c) I do not believe you can get everything you need from one person. It was quite funny the other night, actually. My GF has an online submissive she's been involved with for about 1.5yrs. She loves him to death. It's always caused insecurity in me, because he's so placid and sweet. She always goes to him with her problems, she thinks he's wonderful. In contrast, her and I fight like two wild cats. Until a few months ago, I've always thought that given the chance, she'd be better off with him and she'd choose him over me. Then suddenly a few things have changed. She said to me a month ago that our passion is not a bad thing - e.g. her online sex with him is sweet; whereas our sex is wild and fun. Then a couple of nights ago, she was having an issue with a friend. His response was very placid, very peacekeeping, and she was enraged at his response. I laughed and said... "but this is what you love him for!"
So if she wants a warrior, wild sex, long deep conversations and a thrift store buddy, she has me. If she wants to feel a peaceful, calming influence, she goes to him. If she wants a Domme woman, she comes to me. If she wants a sub boy, she goes to him. If she wants an argument, she goes to her husband. Haha
Me personally? If I want to go on a night out, I take my secondary. My GF hates bars. If I want affection and love, I go to my GF. If I want long, soft, spiritual sex, I go to my secondary. If I want hot and wild and fun and fast, I go to my GF. I can honestly say that currently, I want my girlfriend above all others, for the rest of my life. I do not feel that way about my secondary, even though I care for her greatly and want to rip her clothes off. My girlfriend is the one I'm committed to; monogamous or not. In fact, scratch that, I'm more committed to her because she lets me be me! She laughs when I flirt with women in bars; she doesn't restrict me. I wouldn't swap her for anyone.
The most important thing, in all seriousness, is that if you are committed to each other, you have to be kind to each other. That might mean making compromises when you really don't want to... it might mean that he seems to have more issues than you do. Hopefully you two will communicate and work together as a team to keep a lovely and solid relationship thriving. Good luck!