I'm another one who uses "come" for the verb and "cum", very very sparingly, as the noun. I think I've only used "cum" in one or two of my books, and in both cases the editor left it as was.
When people say they write erotica instead of porn, mostly what I think they're saying is that they write about sex and do it well, as opposed to writing about it poorly.
... But for the record, I say there is a difference between erotica and porn, and don't much care if the 'argument' bores other people. I've explained on my blog before why I take this stance.