Opponents of gay marriage make it clear that they do not object to gay marriage based on a long-range utilitarian calculation; they directly value not allowing gays to marry.

Well, most of them do so in part out of their deity telling them that that's a value. If the extrapolated CEV takes into account that they are just wrong about there being such a deity, it should respond accordingly. (I'm working under the what should not be controversial assumption that the AGI isn't going to find out that in fact there is such a deity hanging around.)