People can already have as many romantic and sexual partners as they want. Polyamory is always an option.

But the legal system is premised on marriage being between two people at a time. It literally can't function if 5 people can marry each other then divorce one at a time, etc.

And the comparison to same-sex marriage is as offensive as it is nonsensical. A same-sex marriage works just the same as an opposite-sex one, and the injustice was denying it to same-sex couples when doing so wouldn't change any straight marriage anywhere.