To be honest, that is something where I see the view. A union between a man and a man (or two women) should have the same taxation etc. as a union between a man and a woman. But a marriage is a religious custom and this is in all religions a thing between a man and a woman.
So remove all taxation (and other) advantages for a marriage and change them so they only apply when a couple (in whichever form) has children.







