Arguments like this really become self-evidently silly very quickly.

Let's just look at a small piece of the irony that evolves from such concepts of atheists getting married.

Why would atheists want to get married for anything other than tax breaks?

We'd have to ask them this:

What is marriage? Who instituted marriage?

Why not just make some vows if that's what they are really wanting to do and then call it something different, like "splunge".

"Yes, we're heading over the the splunge right now."

"We're going to get splunged today!"

"There's nothing like a September splunge!"