Too much of a good thing is apt to be indigestible. I won’t try to define “oversexed” when it come to individual people, but I see all too many stories that I’d class as oversexed. Bear in mind that the operative term here is “I’d class,” so this is all a matter of my own possibly jaded opinion
It’s not exactly a question of too much sex, but of too many actions, “dirty” words, sexual paraphernalia both organic and mechanical, and anatomically improbable acrobatics crammed together with little or no focus to draw the reader into the experience. Gay porn writer Lars Eighner, in his book Elements of Arousal (a fine writing guide even if you’re not writing erotica/porn,) uses the example of a circle-wank scene where one super-stud after another shoots his load, but there’s no one viewpoint character for the reader to identify with. Switching from one viewpoint to another and then another and on and on irritates me just as much, as happens in many an orgy scene, but that may say more about the inflexibility of my aging mind than about the general appeal of such hopping between heads and sets of naughty bits.
There are plenty of other ways for a story to be oversexed. A good story needs pacing, and flow, and varying levels of intensity, with sexual tension that builds toward a literal climax. If there are multiple climaxes, a chance to breathe now and then gives the next wave time to swell. There’s also a tendency to throw so much into the mix at once that the writer has to strain to think of ways to top what’s already been written, sometimes with results that cross over from the stimulating to the ludicrous, or even the baffling. “Wait…she was bent frontwards across a solid table,” the reader thinks. “So how can anyone be biting her breast?” Or the eternal question arises of how many limbs doing what where at which angles can any two people (or three, or whatever) manage? But that one isn’t directly related to too much unrelenting sex.
“Unrelenting” of course brings to mind BDSM and various flavors of power play, but in some ways the elements of dominance and submission and other forms of edge play provide enough of a range of experience besides basic stimulation of sex organs that they may well be over the top, for individual values of where the top is, but not precisely oversexed. (The whole matter of how closely sex and power differentials are related could make for an interesting discussion, full of debate about definitions, but let’s not go there just now.)
It occurs to me that I could just as easily argue against everything I’ve just said. I know perfectly well that a really good writer could make an all-sex-every-instant story work, and that plenty of readers get off enthusiastically on non-stop fucking. “Oversexed” as applied to stories is every bit as much in the mind of the beholder as when applied to people. I don’t have just time now to argue with myself, though. I have to dive back into erotica submissions for the anthologies I’m editing, and try to overcome my urge to skim past any long, long paragraphs packed with sex to see whether there’s any framework of an actual story there.