Why shouldn’t they have it? If the civil law doesn’t outlaw adultery, and people are absolutely free to have as many casual sexual contacts that they want, whether they’re married or not, whether they’re gay or straight, then how in the world can the same civil law tell people that they can’t limit their sexual contacts to a closed set of people. Or for that matter, if adultery is completely legal, as it is, and a man can have sex and make babies with different women at the same time, then how can the civil law tell that same man that it won’t allow him to commit to the several mothers of his children in some legally-recognized arrangement approximately polygamy? it’s over, folks. It all began when the Anglican Communion allowed contraceptives for married couples in 1928. That little acorn contained within itself the entire oak of liberalism. And now we have a full grown oak. We need to quite complaining about it and change the laws to fit what we’ve become.
As bad as it is and much as I do not agree with it, polygamy certainly makes more logical sense to me than so-called “gender-neutral marriage”. At least with one man married to 2 or more women, his children will at least have the benefit of having their father legally married to their mother. That is completely impossible in a same-sex relationship where by necessity any child conceived by IVF or other means will be by definition permanently separated from either his father or mother.