I knew this assertion was stupid to begin with, but I recently had to address this misconception in an argument with a fellow liberal. So for the sake of dispelling the myth, I'm addressing it here.
In all of the state supreme court rulings on the issue of gay marriage, none of them support forcing churches to alter their ceremonies to accommodate gay couples. Such a policy would constitute a violation of the separation of the church and state, but furthermore, when conservatives (for the most part) embrace this canard, they betray an ignorance of the institution of civil marriage, which government is responsible for creating and maintaining.
All of the state supreme court rulings reflect this point.
Links to the rulings are provided below the fold with emphasis on the relevant text.
From the Iowa Supreme Court:
This contrast of opinions in our society largely explains the absence of any religion-based rationale to test the constitutionality of Iowa’s same-sex marriage ban. Our constitution does not permit any branch of government to resolve these types of religious debates and entrusts to courts the task of ensuring government avoids them. See Iowa Const. art. I, § 3 (“The general assembly shall make no law respecting an establishment of religion . . . .”). The statute at issue in this case does not prescribe a definition of marriage for religious institutions. Instead, the statute declares, “Marriage is a civil contract” and then regulates that civil contract. Iowa Code § 595A.1. Thus, in pursuing our task in this case, we proceed as civil judges, far removed from the theological debate of religious clerics, and focus only on the concept of civil marriage and the state licensing system that identifies a limited class of persons entitled to secular rights and benefits associated with civil marriage.
From the Connecticut Supreme Court:
Finally, religious autonomy is not threatened by recognizing the right of same sex couples to marry civilly. Religious freedom will not be jeopardized by the marriage of same sex couples because religious organizations that oppose same sex marriage as irreconcilable with their beliefs will not be required to perform same sex marriages or otherwise to condone same sex marriage or relations. Because, however, marriage is a state sanctioned and state regulated institution, religious objections to same sex marriage cannot play a role in our determination of whether constitutional principles of equal protection mandate same sex marriage.
From the California Supreme Court:
Finally, affording same-sex couples the opportunity to obtain the designation of marriage will not impinge upon the religious freedom of any religious organization, official, or any other person; no religion will be required to change its religious policies or practices with regard to same-sex couples, and no religious officiant will be required to solemnize a marriage in contravention of his or her religious beliefs.
From the Massachusetts Supreme Court:
We begin by considering the nature of civil marriage itself. Simply put, the government creates civil marriage. In Massachusetts, civil marriage is, and since pre-Colonial days has been, precisely what its name implies: a wholly secular institution.... No religious ceremony has ever been required to validate a Massachusetts marriage.
See? All the conservatives have to do is be proficient in Google and reading and they would no longer embrace this myth!
Unfortunately, it appears this is too much to ask, so I have to dispel the myth for them.
Edit: I think movie buff has it right:
Rhetorically speaking, perhaps we'd have a lot to gain by referring to this as "same-sex civil marriage" or "civil marriage equality." In all our discussions, say "I really don't think there's a case for denying gay people the right to civil marriage." The word "marriage" alone seems to freak people out.