I'm fine with polygamists and the guy who wants to marry a park bench pointing to same *** marriage while arguing for their cause.
I'm fine with it as well. It's just that there's reasons for denying those marriages that are specific to those marriages and even if you can find one related aspect, it doesn't change all the other problems.
Pretend (and this isn't directed at you, Bel) I work in a restaurant and come up with the brilliant idea to serve rabbit. I go to the chef, "Hey! We should serve rabbit!"
"Because it's delicious!"
"Well, sure but people won't like the idea of eating rabbit and it's expensive to buy and full of tiny bones so it's not a good fit for us."
"But we can give it a non-rabbity name like Easter Chicken and I knows a guy who raises them cheap and we'll make enough money to hire a guy to debone them for the customers. Come on, this stuff is hella delicious and we should serve it!" (I'm really excited about rabbit)
"Ok, fine. I'm convinced."
So now we're serving rabbit for the main reason that it's delicious after having solved the problems. Three other people come in...
"We should serve panda! It's delicious!"
"We should serve pufferfish! It's delicious!"
"We should serve unicorn! It's delicious!"
"But panda are endangered and illegal to serve, pufferfish are poisonous and unicorns are imaginary. None of those are good idea." "But it's delicious and you serve rabbit because it's delicious so now you HAVE to serve panda and pufferfish and unicorn!! You have no choice because the slippery slope demands it!
And this is why every restaurant that serves rabbit also serves panda, pufferfish and unico--- oh, wait, no they don't. Because finding one shared aspect doesn't actually mean you're going to ignore every other problem.