The answer is that the silent holocaust of assimilation has caused them to disappear as Jews. Intermarriages are twice as likely to end in divorce as same-faith marriages (75% divorce rate! Some reasons for this are the different identities of the spouses and the differences in culture and family.
For example a Jew will naturally turn their head at the mention of "Israel" and "Jew." A gentile who converts in superficial and insincere conversion only for the sake of marriage does not create a new identity that is now Jewish. One is granting a victory to anti-Semites who seek to destroy the Jewish people.
However, in my experience I have seen over and over again that when a child is sees and feels the enormous love that a parent has for them the greater the chances are that they will return to their true ideals and beliefs.
That does not mean, God forbid, that you should let him think that you condone their relationship – you should make it perfectly clear, albeit in a gentle way, that you are completely broken by his actions.
We are trying our best to raise our children as Jews and give them a Jewish education. Otherwise, it's entering a relay race, but choosing a partner who's running towards a different finish line. It outweighs any synagogue or temple, even the Holy Temple built by King Solomon.
Your letter made me very sad and my heart goes out to you at this most difficult of times.
Not knowing your child and his or her personality it is almost impossible for me to say with any certainty what might be the approach for him.
All things are mortal but the Jew; all other nations pass, but he remains. " Intermarriage is a betrayal of our task and of our "choseness." It is also a guarantee against Jewish continuity. "How do you expect your son to follow Judaism when you don't?
Let me illustrate with a conversation heard on the Dr. " Being Jewish isn't a cultural affiliation or a tradition. That means a commitment to the responsibility given to us by Hashem at Sinai.