It is very easy with religion to wonder what the big fuss is about with marrying outside the faith, and as someone who grew up without a religion in the house, who experienced it as an adult and then have migrated into having faith and spiritual beliefs but no particular religion, it can be alien to me people would care. With Jews, maybe because I have been around a lot of Jewish people in my life and I see the way things play out, to me it is a lot more than religion it is also the culture, and if you look at the Jews and their history, all the struggles to maintain their identity in foreign places, the holocaust and its aftermath that wiped out a significant percent of Jews in the world, even pretty liberal Jews, or humanistic or cultural ones, have the preference that who there kids find are within the faith/culture,to try and keep it alive. To be honest, I can also understand the fear because sadly, there is enough anti semitism out there, whether overt or subsconcious, where there can be this wall that happens to somehow block out the Jewish side of the family (to be fair, I also have seen where a person of faith, any kind, marries an atheist, where the religious aspects are shut out, though a bit less common).
With Christians who care, it often boils down IME to the idea that their faith is somehow ‘the only one’, for example, among Catholics that I have seen, the more liberal, ‘cafeteria catholics’ are generally a lot less upset if their kids marry a non catholic than those who consider themselves more traditional. With Muslims and evangelical Christians IME, there are very strong prohibitions against marrying outside not only the faith (IE Islam or Christianity) but their particular version of it (ie Sunni/Shia/Sufti), so it gets complicated (same way that IME orthodox Jews and some conservatives would want the potential in law to be of the same).
I think with religion the part I would struggle with is if the person he felt attracted to had views widely divergent from mine, not about the beliefs themselves, but for example, if they were openly bigoted against people of other faiths, or were very anti semitic or homophobic, it would be hard. I don’t think I would have much problem with people of other races, my son has dated women pretty much exclusively outside our race, and that wasn’t a factor with my wife or I at all, all we cared about was who they were as a person and how they treated our son (we actually are secretly hoping that whoever he finds, keeps him on his toes, think someone like Maureen O’Hara at her best lol).
What I don’t understand is when it leads to total rejection, where they for example ignore grandchildren or treat the grandchildren of the ‘suspect’ relationship badly or worse than their other grandchildren, to be that is an abomination, and if it is over religion that is really pathetic, because children are not responsible for who their parents are, are innocent, and every religion I know of treats them as something special (or should, if they followed their own teachings…). A grandchild to me would be a precious thing, and whether he was part X and part Y, part this and part that, wouldn’t matter, would still be a precious thing, and those who would reject a grandchild based on race or religion has to be pretty stone hearted IMO.