Perfectly acceptable.
Just because it's a social norm to have a monogamous relationship only makes it a better reason to be in an open relationship. People go around talking about society like it's a great thing, but in the end it's just a pile of ants doing the same shit over and over again. You can be a good ant and do what other ants do, or you can be a bad ant and don't. Either way you're going to die eventually... So would you like to be a part of system that will not provide any benefit to you beyond your lifespan aside from a very general unsophisticated survival of the species that still won't personally affect you at all or would you like to be against it, I don't quite give a damn.
You're gonna die, I'm gonna die, and eventually heat death will wipe out the entire universe and everything that's been "accomplished". Do whatever you want.
You can contribute or watch the world burn, on a long enough time span it's all irrelevant.
Or in a less cynical way, there are plenty of shitty traditions that we've overcome now but wouldn't dream of overcoming just a few centuries ago. Just a few centuries ago most people would laugh at the idea that races are pretty much equal. Same thing for any big social change, whether suffrage or gay rights or religion or whatever. Unmarried couple having sex would be a horrible thing in general opinion not even that long ago, and now everyone's doing it and it seems okay. Same for interracial stuff.
I'd say you'd have to try pretty hard to break something hard enough to make the whole card house of civilization to collapse and not have everything, you know, adapt and turn out for the better.
