Shulamit Reinharz, in her forward of "Sexual Violence Against Jewish Women during the Holocaust," suggested that there was a sort of academic taboo against considering that question because people didn't want to be seen as playing the suffering Olympics. That is to say, if you "overemphasize" gendered violence that only effected some victims, you might be seen as under-emphasizing the suffering of the rest. That, together with the natural reluctance of rape survivors to talk about their experiences, together with the fact that the Nazi's didn't document that form of abuse--which was illegal under their racial purity laws--seems to account for the lack of discussion on the topic. It absolutely happened, although given the issues I just mentioned, I'm not aware of any hard numbers documenting how common it was.