It's the only reasonable explanation for not supporting Civil Unions. We've had the debate, the thread is still open if you'd like to present one, but no one could come up with any other reason to support "Gay Marriage" other than slapping at religion. A solution to the problem has been offered, and you've rejected it because you can't steal religious sanctity, which is your ultimate goal and reason for supporting this issue. It's not about helping homosexual couples, who could be realizing benefits right now, with CU legislation.
Liberals want to compare Civil Rights with Gay Marriage, and I think Gay Marriage is more comparable to the failed ERA of the 1970s. Where we, as a society, realized that we didn't need an Amendment, we already have the mechanism in place to address the problem and it was a matter of implementing the mechanism to solve the problem. Perhaps this is why Liberals have such a shallow respect for Civil Rights, they presume it was this issue like Gay Marriage, when it was something much greater. There was no mechanism before 1964 and the Voting Rights Act. There was no way to address the problem. And even in the advent of CRA, it has not solved the problem of racial prejudice in America, that problem still exists.