Menanggapi pepe.

Whilst "morality", etc aren't legal issues, "marriage" is, and it's a union of two people (hmm.. let's not get carried away with whether the two people have to be a man and a woman, that's a separate issue and a potential topic for another debate ).

You can say it's their right, their freedom of choice. It's true, but it will potentially damage others' rights too.
It's one's right probably to have more than one spouse, but what about one's right to wanting just one spouse?
Is there going to be a specialiced institution which deals with this polygamy, researching whether both the parties mutually agree on it? That it's not just one person forcing it to happen (either by financial force or physical threats)?
This won't happen if polygamy is illegal, hence people don't even think about it and probably divorce is the more common option, where then the physical threats will stop and financial support will start.