Since we don't live in that ideal world, our only option is to designate some body to decide what are rights and what are not. Whether that body is a king or a judge or a council of judges or a legislative body or the full populace (or some combination of the above) is an important and worthwhile discussion, but in the end we do need to designate some entity to decide what the rights are.
We've added a few hops, and hopefully increased the average intelligence of the people who get to decide on rights, but they still end up there based on the population's vote.
The campaign for gay marriage, branded as marriage equality, actually leaves many people behind.
Brothers can't marry sisters, mothers can't marry sons (nice way to avoid inheritance tax!), men and women can't join in a polygamous marriage even though there is a long history of polygamy in many cultures around the world.
It's quite ironic that the campaign for marriage equality is itself highly discriminatory.
(which I've never understood why it wasn't spelled "beastiality")
I guess the rationale for specifically bestowing marital rights on same-sex (but non-related) spouses is that there's a significant number of them.
Polygamy, too, there might be a lot of that somewhere in the world, possibly Africa, obscure corners of Utah, certainly pre-modern China when it was normal for a wealthy land owner to have several wives, etc. I suspect it won't be long before polygamy is legalized. Maybe a couple of decades at most.
Like a diet that sometimes prevents people from eating what they crave, the system is nothing more than a mechanism of self-control for the majority.