“Marrying a non-Jew is one of the most cardinal sins a Jew can commit”
Tell THAT to Esther HaMalkah. (I am being facetious, so don’t bombard me with a response). There are three cardinal sins that a Jew should NEVER commit 1) Avoda Zara
2) Shfichus dam and 3) giluy arayos.
A Jew should not marry a non-Jew, but sometimes it happens, chalilah. The idea is to try and keep the lines of communication open. People divorce their spouses, they do teshuva, they change their minds. If they don’t and if the wife is the Jewish partner, all their children are ALSO Jews. Are you prepared to treat a Jewish child as the product of a cardinal sin because his father is a non-Jew? If the husband however is the Jew, there is a greater problem, obviously. But it is not so simple in this day and age to dismissively say “all contact must be broken off,” and “sit shiva for them.” It is unrealistic to think that a Jewish mother in this day and age could easily do that regarding her child. Better to examine what is lacking in our frum society, to cause a Jewish child to seek out non-Jews for socializing purposes.