There is considerable evidence that young nobles had intimate relations with peasant women, but I doubt if sexual gratification was the only reason, and I doubt if force was necessary in most cases. One must remember that, given smallpox, mumps, other diseases, dietary deficiencies, and a number of other factors, a far larger portion of the population than today was sterile. This caused something of a problem, I should think, because the functions of noble marriage -- whether it be a family alliance, descent of property, or something else -- depended on the birth of heirs and heiresses. One way that a young noble could improve his standing in the marriage market was to have proof that he was capable of fathering children, and the best proof of that was that he had done so. I might add that the "common belief" that the wife was responsible for a childless marriage may often have stemmed from the husband's previous proof of his potency.
Another factor entered into such relations. A man with a couple of bastard sons had less to fear from a barren marriage since all he had to do was to recognize them in order to have heirs and successors at hand. This was particularly important since the lack of an heir could always be construed as a sign that a patrimony was to be had for the taking. In the accounts of such relations that I have encountered, the woman received a handsome dowry -- particularly if she had borne a sturdy son -- and the noble married her off as well as possible. The fact that she also had proven herself fertile no doubt made her even more attractive. Except in those few regions where noble descent in both lines was required, this practice offered at least a few women the chance to rise in station. In the case of Hawisa, the mother of William I, the leap from tanner's daughter to vicountess was substantial.
The practice of betrothing young or even infant daughters, as in the case of Petronilla, heiress to Aragon, to Count Ramon Berenguer, was another factor promoting extra-marital relations between noblemen and peasant women. The husband often had to wait some years before the marriage could be consummated, and that consummation was by no means assured since mortality among the young was substantial, for both nobles as well as lesser folk.