There are a few books Piers Anthony has written which do not suffer from being oversexed. I don't mean they don't have sex; I mean, they deal with it better than others. On a Pale Horse actually has a point as a book, and having a point squeezed out a good half of the sex filler that would have otherwise dominated. Bearing an Hour Glass also struck me relatively well, but that could be because it had time travel. In Xanth, Night Mare was good enough on its merits, of having a gripping plot with well-sustained suspense, to overcome the few scenes with sex. It really helps that the main character is a horse.
On the other hand, Piers Anthony has written a few books which though they have a high sex quotient survive somehow. Golem in the Gears is the quest of a very small man to find someone with whom it is possible for him to mate. Considering the normal difficulties he would have finding a partner, it is quite reasonable that his sexual frustration has grown to sufficiently epic proportions that he would go questing. And it has a nifty little math-related section at the end.
Back in the Incarnations of Immortality, the Love of Evil contains much sex, but displays it in the unusual-for-Piers-Anthony light of actually having moral implications. In the end, recall that the main character is Satan after all, so his choices are understandable if not forgiveable.
Aside from those five, I would be hesitant to recommend a book written by Piers Anthony.