My spouse and I had a similar-ish conversation - however, we started off what way back in the mists of time was just called a 'non-exclusive' relationship and only became exclusive after our other relationships naturally ended and we just didn't pick up any others and after living together on what was meant to be a trip, we chose to get married.
Decadeish later, we were stressed out while dealing with so many caring responsibilities both in our home and in our wider family and for some reason my mind kept compulsively going back to our early relationship where I had less but similar stresses but then I also had multiple romantic relationships that I kept thinking about and he said one day I was in bed struggling to sleep and he just rubbed my back and said that maybe the reason I keep thinking about it is I miss that, that maybe it isn't as behind me or on the shelf of my youth as I was thinking prior to that.
That conversation was over 7 years ago and while I've met some nice people and families, but I'm still living as monogamous. We both are. While we both have the capacity to be polyamorous and I have enjoyed that before and might again, but I don't really actively pursue it. I have daydreams now and then about what I would ideally like and we talk about them, but I'm in a 'if it happens, it happens' space as I was when I was younger. I actually found many of the polyamory-specific space really off-putting.
We do have a friend who lives with us who has been called a platonic partner of the household and was largely accepted by our extended family before their deaths - he went to several family meals in our area and he even went and was welcomed into one of the funerals. I've also had affectionate friends - as in holding hands rather than as a euphemism, though they've moved on. I've had crushes which I've discussed with my spouse, but there wasn't really enough there to pursue.
Honestly, I think part of the appeal of polyamory - and where some of the community issues stem from -- is that as a society we've built up a narrative around romantic and sexual relationships as the most important barring possibly children, with familial coming after, and then friends and then maybe wider community and with how our economic and other social systems system are structured, our free time becomes narrowed, many lose friends or have friendships become most distant over other commitments, and while being called friendless is an insult, it's rarely taken seriously, even less seriously considered are the emotions within a platonic relationship having value.
I think it is currently more socially acceptable right now to say 'something was missing/wrong in my marriage so we decided to open it up' than it is to say 'I am lonely'. I think, as a bisexual women married to a man, that it's more acceptable and less emotionally vulnerable for me to say 'I miss having romantic and sexual relationships with women' than it is for me to say how much I miss the friendships I've lost over the years and I'm not sure how to rebuild my social life. I think socially we've made it harder for adults to look for friends than friends "with benefits" (as if friendship isn't enough of a benefit). For many, yes, multiple romantic and/or sexual relationships is enjoyable, but I think just as many people within polyamorous spaces are looking for more human connections and this is a route they've found for it. That there is a large asexual community within many polyamorous spaces I think speaks volumes.
Ethically, the more people, the more ethical issues that can crop up and I do think some polyamorous communities need to own up to the fact that some of their ideals like 'no rules, only boundaries' and similar has been abused and that many of them perpetuate the same type of harmful dating ideals as wider society. The tendency of those in power within those groups to use softer words around it or assuming just because someone consents that there isn't an issue or the whole 'don't yuck someone's yum' as if power dynamic issues don't affect us because we can love multiple people and use certain words has really put me off being in those spaces where they're in charge.
If one person can't meet your needs then there is no point in getting married. So your marriage isnt worth the paper its written on. Its a lie.
Marriage certificates are just as legally binding in this situation. Not everyone includes 'forsake all others' or similar in their vows and while adultry is a long legally accepted reason for divorce (though not for ending civil partnerships), it doesn't actually make the document a lie anymore than it would so a will.
The Romantics might have built up this cultural ideal of one person meeting all our needs, but that's never been the case and even in religious traditions where adultery is a major sin, the idea of one partner has largely been held to one sex and other platonic relationships are still a thing. We're not as independent as society tries to sell us is good, our needs for others runs deeper than that.