The Wehrmacht committed many many acts of rape and sexual enslavement on the Eastern Front. We are all aware of the "rape of Germany" by the Red Army because of the Beevor book (Berlin: the Downfall) and it is popularly assumed that the Russians did a lot more raping than the Germans, but that is a misconception based on the lack of information on the sexual atrocities in the Soviet Union.

It is notoriously difficult to gather accurate information, let alone statistics, about sexual violence because of the many taboos surrounding the subject, the stigmatising of the victims, issues of female and family honour, the shame felt by victims, the fact that at the time the fact finders and government officials were male and less attuned to female-specific consequences of the occupation and simply did not record such crimes, and a whole host of other factors. Therefore, we will never know the true extent of the situation and we largely have to rely on eyewitness testimonies.

A complicating factor specific to the Nazi context is that any sexual contact with Slavic women, and even more so in the case of Jewish women, was considered "Rassenschande" (racial defilement) and officially prohibited. This led to additional secrecy, denial, avoidance of the subject by superior officers, etc.

Nevertheless, it is pretty clear that sexual violence was extremely widespread in the occupied Soviet Union, in accordance with the general extremely brutal nature of the occupation in the East (as opposed to the occupation in Western Europe).

For further reading, I can heartily recommend W. J. Gertjejanssen, “Victims, Heroes, Survivors: Sexual Violence on the Eastern Front during World War II,” University of Minnesota, 2004. This dissertation can be downloaded for free from the author's website.

S. Neitzel and H. Welzer (2012), Soldaten: on fighting, killing, and dying : the secret World War II transcripts of German POWs has many quotes from the secretly recorded German POWs that are rather hair-raising.

"Estimates of rape victims from the city's two main hospitals ranged from 95,000 to 130,000. One doctor deduced that out of approximately 100,000 women raped in the city, some 10,000 died as a result, mostly from suicide. The death rate was thought to have been much higher among the 1.4 million estimated victims in East Prussia, Pomerania and Silesia. Altogether at least two million German women are thought to have been raped, and a substantial minority, if not a majority, appear to have suffered multiple rape." From this article http://www.theguardian.com/books/2002/may/01/news.features11 about a book covering it. Women would also try to offer themselves to just one soldier in order for them to protect them from the others which is certainly a form of sexual slavery. I don't know the numbers on how many did this though. Needless to say it was extremely widespread and Aoviet leadership did little to nothing to stop it in the early parts of occupation. Stalin himself actually more or less came out in defense of it.