We might think of this on three levels. First, is it permissible for a liberal state to outlaw necrophilia? The argument for an affirmative answer could appeal to various public health reasons as well as to the fact that this practice may give considerable offense to others even while the cost of abstention is relatively small and borne only by a few. This argument might run roughly parallel to that justifying the permissibility of outlawing nudity or defecation in public places. The case of gay relationships is substantially different for two reasons: the cost to gay people of not having the opportunity of a romantically fulfilling and socially recognized relationship is enormous and, with roughly three percent of all people being gay, the number of people who would be (and have been) bearing this cost is substantial.

Second, is there something ethically wrong with practicing necrophelia? Taking ethics in the broad sense, its concern is the good life for human beings. A good life centrally involves close personal ties, friendships, and romantic relationships with people who we regard as our equals and with whom we engage in a wide range of communicative interactions. Compared to such interactions, necrophilia is an inferior activity, a waste of time. But so are many video games and TV shows. And it's surely not a serious failing for people to take a little time out here and there for something dull or silly.

Third, is it more narrowly morally wrong to practice necrophelia, would doing so wrong other people? Setting aside the wrong one might do to others by endangering public health and/or by violating the laws of our common legal system, and assuming the free and informed prior consent of the person now deceased, it is hard to see who might be wronged if the act is performed in a private setting.

I agree then with what you are suggesting: the strength of our reaction to necrophelia cannot be explained by reference to our modern moral-ethical thinking. It is presumably related to religious commitments, aesthetic tastes, and even biological responses.