Absolutely! Changing the rules mid-stream has to be mutual in order for it to work. Even though James and I both agreed at the outset, there were things that happened when we opened up that we hadn't anticipated and we had to work through that together. Fortunately, the high level of trust. and communication we had helped us get through it successfully. There are many young people (and others) these days who have never bought into monogamy at all though, and for them, it's just a matter of connecting with people who are of like mind. This is a much better situation that starting with one relationship style and then trying to learn another one, because there is a significant learning curve when you do that. But, some of us didn't really understand that we had any other options until later in life. ; D
Earlier today I saw on Twitter a thread about why some people who claim to be poly end up going back to monogamy at some point. The answers confirmed my point of view. Poly takes a lot more work than many people are used to putting into their relationships and some just get tired of the effort. Either that, or they've never made the effort in the first place and are just trying to essentially do monogamy with more people, which isn't going to work. Monogamy is easier because everyone already knows "the rules" where in poly you have to negotiate the parameters with the individuals involved.
There certainly are people who open up and then find that it wrecks what they have, although I don't know the numbers. My impression is that this happens more often with swinging than poly. It's not a relationship crisis for me (or James) to fall in love with someone new, where it likely would be if we were just swingers.
Whether or not you would require the consent of both your wife and a second lover if you wanted a third lover is not a given. It would all depend on what you had negotiated with both of them already. You might well have a relationship hierarchy where your wife gets a say but a secondary relationship doesn't, or you might have "relationship anarchy" where the only parameters are the ones that you and that other third person have agreed upon and all others get no real say. There's an infinite variety of combinations of all of this depending on how many partners you have and what the level of commitment is. This is what I mean when I say it's an entirely different relationship style. Is it a deep and committed love relationship or is this other person someone you really care about but it's more casual and really more about the sex? Since both of those, plus all sorts of other variations on affection and sexual involvement are possible, the variables are nearly infinite. See why some people just would rather throw up their hands and go back to a situation where they don't have to work that hard because the rules are already figured out for them?
I do think that default monogamy means that most people fall into a bunch of rules and roles that they have never thought much about and that we'd all do better to think about what we want and need and to express that before starting any new relationship of any sort. There are plenty of people who go about open relationships without truly understanding all the elements of self-responsibility and clear communication, etc. I think everyone would do better to embrace the honesty, and other ideals of polyamory, no matter their relationship style. But like I said, that's work that many people just aren't up for.