Not to sure about that, to the medieval mind such encounters were often a sign of coupling with satanic forces.
Wiki clicky.
The article is slightly wrong about the Malleus, the book states that an incubus and succubus were two facets of the same demon.
See me, see reading too much p1sh.
Oh aye, and the correct translation of Malleus Maleficarum is " Hammer of Evil"