Like my colleague, I agree that you've put your finger on a potential moral problem with necrophilia. However, I have a worry about the rest of his analysis. My colleague writes: "I think the more primordial objection to necrophilia is that most of us see the good of sexual intimacy as the loving union of two persons." I certainly don't disagree that this is a good of sexual intimacy, but aside from the question of whether it's the good, I have a different worry: even if the loving union of two people is the good of sexual intimacy, it wouldn't follow that other forms of sexual activity are morally suspect. The most obvious case is masturbation. I've never seen anything that struck me as a plausible argument that masturbation is wrong. A standard Catholic objection is that all sex acts should be "open to the possibility of procreation," as some Vatican documents put it. My own view is that this is bad as theology, and worse if one doesn't accept theological premises. Another objection is that it's a special case of using someone—oneself in this case— as a mere means, and thereby not respecting their humanity. I think the right reply is that people masturbate, among other reasons, because it feels good, and for the release it brings, and it's a highly implausible stretch to say that people who masturbate don't respect themselves. And so if there's something morally wrong with necrophilia apart from the possible consequences for the wishes of living people, we'll need a different account.
Suppose it turned out that the practice of necrophilia tended to undermine one's ability to form relationships (not necessarily just sexual relationships) with other people. Suppose it tended to desensitize people who practice it in ways that undermine their moral sensitivity. I don't know whether this is true, and I don't think my hunches are worth anything. But certain habits and practices surely do tend to diminish one's moral responses, and if necrophilia is one of them, that would be a problem. But I stress: I don't pretend to know that this is true.
Another potential problem is that someone seeking out the opportunity to have sex with a corpse may very well need to cross other boundaries just to bring the situation about—breaking into a morgue, for example.
My first thought when I read your question, however, wasn't about the moral wrongness of necrophilia but about what Aristotle called eudaimonia—with human flourishing. I wonder whether someone who practices necrophilia is likely to be stunted in other ways. It's not altogether easy to see necrophilia fitting into an otherwise thriving psychology. One again, I don't think my mere intuitions here are worth much, but it's hard simply to set those intuitions aside.