The issue of 'adult' content in d20/OGL products has recently become something of a 'hot potato'. When Mongoose released Nymphology, some months ago, this by no means the case, and I don't recall the product generating much in the way of controversy (though it is possible my memory has simply been eclipsed by the more recent events).
Nymphology is intended to be three things: humorous, adult, and a gaming supplement.
The author begins by specifying that the book is intended to be played for laughs. It's worth noting that these are laughs of the kind you get in movies like 'Dumb or Dumber', or 'Revenge of the Nerds'. It's not subtle, and it's sometimes crass. Some of it is genuinely funny, but how much of it raises a laugh will vary widely, depending on the reader.
As for the adult content ... well, it definitely doesn’t meet the current rules for the d20 license, but if you're buying it for the rude bits, I think you'd be better off with something from the local newsagent, instead.
So it's more-or-less amusing, and mildly titillating: but is it any good as a gaming book?
Nymphology begins with an overview of the subject of 'blue magic' - magic used for physical (and possibly romantic, but mainly physical - gratification. It discusses how each of the standard schools of wizardry can be applied to 'blue magic', how 'blue magic' gets learnt and taught, and the attitude of 'typical' wizards to blue mages (and vice versa). This section is actually quite well thought through, with some interesting observations on the potential uses and misuses of 'blue magic', and the effects on both the caster and the subjects.
We then have a short chapter discussing 'alternative' uses of existing spells - you'll never look at grease the same way again - ranging from the obvious (you can use planar binding spells to get yourself a sexual partner? Who would have thought?) to the relatively subtle (wizards who hold regular false vision shows, and charge subscribers to scry them).
There are also a fair number of new spells. If people really could do magic, these are probably the first spells they'd learn. They vary from mildly amusing to ridiculous to wincingly crass. A small number might actually have some non-sexual uses, but the focus is very much on body parts and their intersection.
The book also discusses the Plane of Lust, where everything is eternally suggestive, and the many denizens are ready to 'get busy' with any passing planewalker. I find it hard to imagine anyone seriously using this in their game, and I suspect any attempt would be doomed to failure. It's essentially a setting where everything and everyone is an excuse for innuendo. Or something even less subtle than innuendo. This is a chapter that seems more for reading and sniggering like a schoolboy over, rather than actually using.
The monster section - at the end of the book - is similarly over the top. If you take every animal related euphemism for a sexual organ, and try to stat them for D&D, you've essentially got what has been done here.
Squeezed between these two sections are a handful of prestige classes. As a gaming product, this is one of the stronger sections of the book. An 'agony mage' with the flavour text changed would probably make for a very workable character, as they are highly skilled at making others trust and respect them: they could be the sly power behind the scenes, for instance. Some careful attention would need to be paid to the balance of the classes used, however. For instance, the 'seducer' has the ability to pay a small XP cost, and permanently charm any target whom they have previously seduced, apparently without recourse to any saving throw.
Overall, Nymphology is most successful as a work of humour. Its gaming value is marginal, unless you enjoy a great deal of smutty humour and silliness in your games, and the score above reflects this.