I'm in a not totally dissimilar situation only I have been married a lot longer and my children are now at college. I got a lot of flak when I broached the subject of having an open marriage or other options.
I don't know why, really. Everyone seemed to be taking the side of my husband. But the fact is, he doesn't want to change the situation. He wants to remain married to me. Like you, I have been quite surprised and a bit amazed by his reactions as if he had told me that he was feeling trapped in the marriage (which is pretty much what I have told him) then I would do something about it.
Or, to put it another way, if I felt HE wasn't happy in the marriage I would actively find ways to improve the situation whether that meant - open marriage/separation/affair (yes, I have told him he can have an affair but he doesn't want one!)
I am nervous about divorce - there are huge financial implication plus both our families are extremely negative about divorce and his family has religious convictions over divorce - as far as they are concerned it is just something you do not do, whatever the circumstances.
The other aspect of it is that I do not want to leave the family home. I just don't. Neither does he.
The one thing that I have considered doing, when I have moments of thinking that I can't go on like this, is drawing up a legal separation so that we could stay in the house but our relationship would be as co-parents/house mates, at least for a time.
That might allow us both some space to see what we both want. And the grass may very well not be greener, as people pointed out before. In any case, the very last thing I want to do is get into another monogamous relationship or - God forbid - another marriage. No thanks! Been there, done it, got the T-shirt and once is more than enough.
For me, I think the only thing that will work is couples' counselling where I can put my side of things in front of an impartial person so that H can really try to understand where I am coming from. No doubt he has things he too would like to say - I think that because of his religious upbringing he sees separation/divorce as a failure so has huge issues around that. Plus his mother was a bit of a martyr and ran around after his father, always putting his needs first (that was kind of the default mode in those days though, to be fair).