I will rewatch Eyes wide, saw it a long time ago not exactly at my senses full capacity.
I think Monogamy is a very unnatural concept - one thats only kept due to consumerism and money. The wedding industry is a billion dollar behemoth, and it encourages consuming and procreation which in turn leads to more spending, thus more money in circulation, thus ways for the empowered to stay in power.
The cheat day idea is not bad, but I'm not sure how it will work in practice.. You know how fickle humans can be, and maybe one would be alright with it for starters, but jealousy would come into play later. I do encourage being more open in the relationship though - especially I would imagine as the flames die down and you become more accustomed to / used to each other, as I think that will keep those flames burning longer. I for one gotta get out of the relationship as soon as I get bored of the person. I have a sneaking suspension that people in 'succesful' marriages usually cheat once in a while. That's what I have gleamed from talking to dudes at least, and i'm sure woman is the same way.
I'm not sure I could consent to it, but if it happens, I'm non the wiser, and it keeps the SO happy.. ey, maybe it isn't such a bad thing. I haven't been in a lot of proper relationships though, so i might not be the best one to ask. I do think the whole cheating thing is vastly overhyped... It's just sex after all. You can still love the other person dearly, but just need something else.. I guess sex is seen as the deeply spiritual thing you share with your SO, but damn man, we aren't living in a movie here.. sex is disgusting but gratifying, and it shouldn't be the basis of ones relationship in my experience, if so, it will die quickly. In short, yes I think having sex with others is healthy for a relationship. Or maybe relationships is meant to be a ships passing in the night sort of deal. Let that sh1t burn hard and bright, then fade away. That's how mine has been anyway.
Back in the day in caveman times, there where no such things as relationships, if the cavebro wanted snu-snu, you made snu-snu, and the whole tribe lived together, the woman raised the children and the men hunted and build things. I think that's the most natural way to live, and the way the human brain is still programmed to live to this day. Bear in mind, this mechanic only changed tops 3-4000 years ago.