The Book of Erotic Fantasy, from what I remember, mostly had sex as a power-up for combat abilities. Which is missing the point in more ways than one, IMO !
Also, much of it is generally considered unbalanced by the d20 experts, which is weird, since Lunar Sage was complaining about overpowered characters.
Doesn't matter either way, if you want any of the flavour in it, you can adapt it to GURPS easily .
Oh, the BoEF is very unbalanced, but not overpowered, it's tilted too far the other way - useless material for the sake of being 'sexy' (exception being the atrocity that is the Metaphysical Spellshaper). I like to say that it follows the Rule of Three: Stuff in that book is Balanced, Mature, or Usable in an actual game; you can pick two, but nothing is all three at once.
Its stuff about breeding/pregnancy and relationships with various races is the real gold mine, plus the single best quote of any D&D sourcebook I've ever read, "Love Life of an Ooze".