Grrr, I typed out a whole reply to both of the above posts and then lost it! Anyway, to recap...we have done the couple's counseling, but found (and had confirmed by 2 counselors) that we already have excellent communication skills and it's not a matter of misunderstandings or difficulties expressing our needs.
In terms of having a "gun to my head", in the past, yes, that's exactly how it felt. But at this point, I think it's more a combination of he has done the work on his own and in therapy to fully explore himself and his motivations. He has come to the conclusion that he is truly mono and really wants (needs even?) a mono marriage. Mix that with the exhaustion of struggling with this constantly for the last year, and you get the idea that perhaps we aren't cut out to stay with each other for life. However, he supports and encourages that idea that I should be able to have that same understanding and confidence in myself before we make any definite moves, which is why the ball is currently in my court.
I plan to absolutely see a therapist in the coming weeks to help me work out and understand myself fully, and to gain the courage and strength to stick to whatever convictions/conclusions I reach. I have a lifelong history of relinquishing my own wants and needs in favor of pleasing the people I care about (I have done this with parents, friends, classmates, and of course romantic partners). I have improved significantly in this area, but only when the answer was obvious or the situation became toxic, neither of which feels to be the case now, hence my fear and indecision.
Finally, I didn't write about all of the steps we've taken in my original post, but we've done many things from just flirting online to the full individual sex thing. Of course, some were tolerated better than others, but all ultimately had the same negative feelings in varying degrees on his part. Again, there is a jealousy component, but I whole-heartedly feel that his reasons for feeling the way about being mono, for him personally, are justified and understandable...I would just rather not get into those reasons here. Suffice to say that based on his life up to this point, sex, intimacy and romantic involvements have taken on significant meanings for him that we simply don't share. We can each see the other's view, and even support the right for each to feel the way we do...we just can't seem to find a way to make it work together. If there are other exercises/steps/etc that anyone here can suggest, I'd be very happy to hear about it. Some may not be viable, but I want to know that I have literally done everything in my power to make this marriage work, while ensuring that everyone ultimately has their needs met. Perhaps separation would be a next step, to facilitate a break (we've taken breaks from discussion, activites, etc quite a few times throughout this process but never physical separation), but I don't know that it would be viable to reconnect if I allowed myself to explore/participate in any outside relationships during that time.
But yes, therapy is definitely on the table, and depending on how individual sessions go, we may even give the couple's thing another go. Thank you both for your comments and advice, I look forward to hearing more ideas and reflections on my situation.