@Violett_Queen I don't think it's already the case and we already generally do just do whatever we want, but are afraid to admit it to ourselves.
Also, are you talking about *gamy or *amory?
There's distinction.
Polygamy is about just sex pretty much. Polyamory is about romantic relationships.
If the former, then, a lot of married people have sex outside marriage. It's debatable how honest this is, most of it just cheating - but that already counts as polygamy.
If the latter... Unfortunately, we're ways off universally recognizing this type of relationship. For shame.
There's also the factor of polyamorous relationships being harder to maintain, requiring a lot more work to do properly. Despite what conservatives might say, this type of relationship demands MORE responsibility from all involved, not less. So, it's not everyone's cup of tea, and probably monoamory will be the default in most of the cases. But if you are willing and able pull it off, there's nothing quite like it.