A jewish woman can marry anyone and all her children will be Jewish.
But that would then only be "half-jews", wouldn't they? ;)
The race laws work here also...there are always big discussions between the rabbis, even today "who is a jew"? Intermarriage with non-Jews seen as biggest danger to world jewry!
Fact is, had the Jews intermingled with the native Europeans that much they would had stopped to be Jews.
In the past, Jewish intermarriage was rare.
It generally occurred when a Jew deliberately rejected his/her religion and heritage and sought to assimilate into Christian society.
In response to this rejection, the Jewish community disowned intermarried Jews. It was common for Jewish parents to sit shiva (Jewish mourning ritual) for children who intermarried.
It wasn't for nothing that even after many centuries of living side by side, even experiencing times of nearly full assimilation, accepted integration, Hitler was able to pluck most of them out of the native Germans at the first try...