marriage is nothing more than a social contract. there has never been one standardized format for this either. go around the world and you will see all sorts of marriages. go back in your own family 2-3 generations and you’ll see stuff that wouldn’t work too well today. at the end of the day, they are social contracts - if the society has legalized it, it is real.
adoption isnt the same thing as marriage. why do single adults adopt kids made by two people having sex? is that even an argument? one adopts to help raise a kid who otherwise might not get that great a chance at growing up happy.