I don't see how that could be true. Many non-religious people are routinely married, legally, and it's called marriage. Prohibiting gays from being married doesn't change that, so if that is the reason, it's irrational.
They already can, so this change is neutral on that issue.
If someone is very religous, they should be quite happy with the freedom to both legally join (whatever the word), and to be MARRIED in say, the catholic church. They can tell people, "We were married in the catholic tradition", or whatever, and it would be true, and would have whatever meaning it has in that subculture or the wider nation. The idea that this would be insufficient, seems absurd.
Fishstyx just proposed this as an alternative. If it's pros aren't want is desired by the parties, then as megaprogram points out, fine, just allow anyone to be married. If it's government, it can't be religious, end of discussion right?