Muslim culture does not consider it to be acceptable, but that doesn't mean it doesn't happen. Aren't you being disingenuous to suggest otherwise? The same can be said of any culture in that not everyone follows the rules. My experience in Morocco the women who were groped were scantily dressed, for example wearing a bikini top and shorts in the street. Those women were not considered respectable and treated in a disrespectful way by some men, but not by most because the local people were too well mannered to do otherwise. Even though I was a young woman at the time, I would not wear a bikini top in the street, especially when I knew that it would be considered inappropriate by the local community. I was brought up to know better.
Like it or not, countries do have different standards of decency and we shouldn't expect them to adapt to ours.
What happened in Germany is unacceptable, but it is not clear what is behind the attacks. There is a lot to be considered, particularly regarding the Cologne incident where the police appear to have seriously misjudged the situation earlier in the evening. It is also possible the attacks were politically motivated and probably carried out by the same gangs of criminals who operate in the area at other times of the day.
I think there's more to this than meets the eye.