Volokh seems to agree with me on the issue of legislating morality (or what people call that, anyway). His fundamental claim:
... religious people are entitled to try to enact their moral views (which stem from their religious views) into law, just as secular people are entitled to try to enact their moral views (which stem from their secular, but generally equally unprovable, moral axioms) into law.
Update: A Physicist's Perspective has more.
In his response to a nut who thinks people want to outlaw being homosexual (which Volokh noticeably avoids commenting on), he argues that even views that we almost all agree on just involve appeal to some moral intuition, and the cases where we disagree stem from genuine moral disagreements on the level of intuitions. It's helpful to see how this is done in specific cases, and he explains how it goes for quite a few issues.
I think there are other ways to argue against the kind of policy opponents of the religious right hate, but you can't do it in terms of not allowing people to legislate based on morality, not unless you're going to redefine 'morality' as not just ethical reasoning in general but religious motivations with no civil value (of course there will be debate over what has civil value, but that's in the realm of ethics).