First of all, thank you to everyone who replied, for your kind welcome, and helpful advice.
Firstly: Quath and Kevin T, you were right about NRE.
I mentioned in my original post that i was crushing on this guy at work. Lets call him 'C'. I spent weeks going on and on about how wonderful i think he is, how much i love his voice and his hair and all that stuff. Then about 2 weeks ago i found out that, due to work schedules, I probably wouldn't be seeing C again, at least not regularly. I came home fairly unhappy, and sought comfort from my Hubby. But he snapped at me, something along the lines of "I'm not going to comfort *my* husband because he's crushing on some guy from work". I began to worry he wasn't open to my being romantically involved with other people afterall.
I now realize Hubby acted that way because I have indeed made hm feel "sidelined". Like you said, Kevin, i thought of it as sharing, when it was really just insensitive 'rhapsodizing' about someone new. Even without the returned feelings needed for a relationship, it was pretty much 'NRE'.
For all my worry about Hubby accepting poly, I hadn't thought about the feelings that would actually lead him to accept, or reject, a new relationship, and a new person.
So i had a talk with him, and apologised for the way i'd behaved. I went on to explain how I was worried, and asked him if he was still ok with my being non-monogamous.
Magdlyn, you were right: Hubby explained that he wasn't against poly per se, but the reason he snapped about C is because he's scared that i might love C, or someone else, more than I loved him, and maybe even leave him for them. Probably because i'd been so enthusiastic about C in the weeks before. I reassured Hubby that i wouldn't leave him, and he said he knows that, intellectually, but that the heart don't always match the head.
I mentioned that the veto thing was, long term, untenable, because it'd hurt me, and the other person, if he vetoed a relationship. He said he wouldn't care about their feelings, only mine, which raised a red flag. So I pointed out that i would care, because i might love them the same as i loved him. Then i was, probably for the first time, frank about what i wanted in my future. I talked about, for example, if me and C had hit it off: How would Hubby handle me asking to date C, or how would he feel if C wanted to move in with me/us? How would he feel seeing C at the breakfast table, or me giving C a kiss goodbye in the morning?
I've never been so blunt about it before. I normally try to be less 'in his face' about it, but i think the vagueness adds to his anxiety. Having concrete situations to consider might have helped him get his head around it more easily, since he surprised me by saying that, in that event,he'd want to meet the person, and if he was going to share such a big part of his life with them, he'd have to like them too. I said that, although people are different and don't always get on,iy was fair enough to expect them to at least not hate one another if i wasn't to wind up leading separate lives with each of them.
Like i said, me and C is probably never going to happen. But you never know with love. For all I know, I might bump into the next special person tomorrow at the grocery store, or mall, or whatever. If it happens, i want to be able to navigate it so as to build a new relationship with the existing one intact.
So thanks for the welcome and the advice. It's so wonderful to have people with experience who can point out when i'm doing something crazily self destructive. I'll probably be bothering you all again on other sections of this forum for more advice in future, and hopefully making some friends.