Oh, I see now. I was compartmentalizing pretty heavily there, wasn't I? I think I know why: the hypothetical situation I was imagining for necrophilia was on a desert island, (probably borrowed from the default one for cannibalism). The hypothetical for grave-desecration was spray-painting a gravestone in a local cemetery. People are less likely to find out in the former, so I never took those considerations into account.

Yes, in situations where grave-desecrating in general is not okay, necrophilia isn't either. I still think both are mostly okay if nobody finds out, and my saying this shouldn't make anyone sad as I have no desire to do either.