I guess that's why atheists never get married...
Why do atheists eat bagels and lox if they are not Jewish? It tastes good, that's why.
Marriage is a religious institution. It also happens to be a very attractive institution to some, as it portrays family, stability and bonding in a way that just being "in a relationship" cannot. If an Atheist envies that and wants in on it, that's fine by me, let them do their own marriage-like thing, but don't try to change the definition of marriage.