It may change society but as to wether it would be a change for the worse is something that can only be subjectively assessed after the change has happened.

As a social mechanism you have to ask what is it for?

How does it function?

It clearly isn't just about producing children otherwise we wouldn't allow people who are for some reason incapable of producing children to marry.

The law as it stands in the UK is that same sex civil unions have all the civil and legal benefits and responsibilities of a heterosexual civil marriage but can't call themselves married which seems daft as they are legally joined in the same way.

Union and marriage mean basically the same thing so if the gay rights people are hot under the collar about not having access to that word, just give it to them.

As for extended marriages to groups of people larger than two I have no problem with it ethically but it may be a legal minefield without well thought out legislation.

I can't see why the act of civil union can't be extended to people who don't have any desire to have sex with each other, like siblings.

That way if siblings living together in a shared home would have the same protection in law if one of them died as a husband and wife would.

I also have no problem with childless siblings adopting children as a couple.

There seems to me an unhealthy obsession with sex in most societies and not enough focus on social structures.