We've been discussing this on a couple of lists. Of course, as I am fond of saying, everyone draws the line somewhere, and that line is never going to match, but when I hear people saying that erotica MUST involve a relationship and that's the line between erotica and porn, I feel the need to respond.
That definition would be wrong. Sorry... There is no requirement for a relationship in erotica. It can be a one-night stand or f*** buddies and be erotica. The difference, really... Well, there are several.
Erotica is meant to arouse the reader. That does NOT have to be accomplished through sex scenes. There is erotica that is more suggestive in that arousal than outright sex. Porn will ALWAYS have very detailed sex scenes. Erotica CAN be used as beat off material. Porn is meant to be used that way.
Erotica USUALLY contains respect between the characters. Porn USUALLY (not always) includes debasement, sexual blackmail or one of the other baser dark fantasies of the (usually) male mind. No offense to our guys. Not all men fantasize these things, and some women do.
Erotica usually contains a strong plotline and characterization. People do not have sex without a reason to have sex. It's revealing something, causing growth or complication, or otherwise driving the story and characterization further. Porn often contains sex for sex's sake. Okay, the EPPIE...I believe, uses that as part of the definition of erotica, but the truth is...it's really not that cut and dried.
Is there such a thing as too much sex? Yes...at the point you are boring your reader, there is too much sex. At the point where you've gotten repetitive, there is too much sex. At the point where you've overstepped the genre you're shooting for, there is too much sex for that genre.
Which brings us to... Can there be gratuitous sex in erotica? Of course, there can. A well-written erotica book/story, as I noted, has a reason for the sex scenes. EVERY scene in a book should have a reason for existing. The scene should advance the plot and/or the characterization. If it doesn't do either, it's an interruption to the natural flow of the book. THAT is never good. ANY scene that doesn't advance the book is gratuitous. Sex scenes can be. Violence can be. Someone baking a pie or brushing teeth can be.
Sex added to a book or story JUST to add sex will feel slapped in to the reader, unless you are very good at weaving in a reason to add it. If you can't do the latter, don't do the former. Just my opinion.