This is a gross mistranslation of the word “niddah.”  

The word “niddah” means “separated,” not “filth.”  It is used most often with regarding women who are menstruating.  The Torah prohibits a man from having sexual relations with a woman who menstruated until she immerses herself in a ritual bath, called a mikvah.  During the time that a wife is forbidden to her husband, she is described as a “niddah.”  There is an entire tractate of the Talmud, Niddah, that deals with this topic.

During the Hasmonean era, the rabbis at the time decreed that all non-Jewish women have the status of a “niddah.”  The sole purpose of this decree was to stem the rising tide of assimilation and intermarriage.  They were hoping that by decreeing the status of a niddah on all non-Jewish women, this would cause Jewish men to give greater consideration to the prohibitions on intermarriage.  

But the word does not mean “filth.”  If it did, then Jews would calling their own wives “filth” every month — and that is not what we are doing.  It simply means “separated,” nothing more.