Step Right up, and take your swing at this wonderfull theory!
I think of witches as a different divergent evolution of humans(Homo Arcana , if you will.), so its perfectly possible for them to interbreed with humans, but any likely offspring probally would be sterile, or become a cornucopia of mix-match traits. This is not even getting into all of the possible magical things that could happen to such a hybrid, if any( The child may become magicaly sterile, or uncontrolled powers[yikes!]). Add to the fact that its been proven that witches can live hundred years, if not thousands, based on implied content, and any possible witch-human relationship would be very short term on the witches part, which would be problematic if a witch despartely falls in love with a human and has to watch them wither and die while they stick around. Of course, a witch could just magic up a solution to any relationship woes, but that's only a possibility. And of course, what of the children?!
I digress, though. My point is that, yes, witch-human relationships are perfectly possible, but very unlikely to happen, due to the reasons above. Either that, or magic. Cause magic is the answer to any problem a witch can face...