"Do you share the same primary values?" I asked a young person who had approached me for advice about marriage. "For example, are you both Christians from the same denomination?"
The young person assured me that they did and that they were. This sounded satisfactory although I must admit that this does not always matter. (In this case it would, though.) If you take a nominal Catholic whose real core values are vegetarianism and earning money and a nominal Presbyterian whose real core values are also vegetarianism and earning money, then there is less likely to be marital friction and disappointment, however their devout or orange sash-wearing parents may feel about it.
It feels a bit risqué to encourage nominal Catholics to just admit to themselves that their faith is not really a core value and to look for fellow adherents to their real religion (money, animals, Gaia) when seeking a marriage partner. I will make up for this by pointing out that religious Catholics are often much more interested in actual marriage than nominal Christians are. Of course, it is extremely unfair for marriage-minded nominal Christians to go seeking spouses amongst devout Christians--unless, of course, they have an inner voice telling them that they really need to do that.
Happily, earning money, vegetarianism, love of animals and stewardship of the earth are not incompatible with Christianity.
Anyway, I know of perfectly happy "mixed marriages" which for Catholics has always meant religiously mixed, not biracial, and of marriages in which the Protestant or agnostic spouse has eventually converted to Catholicism. Presumably there were other core values that brought them together--or kept them together--all those years: marriage itself, mutual love of their children, etc.
I deeply believe that when you know and embrace what your core values are (pick the two deepest), you should not settle for different ones in a life partner. (Marriage is hard enough. Really.) You should also listen when you get a crush on a handsome or beautiful person and they level with you that they can never marry someone like you because, for example, they must/want to marry someone from their own ethnic background.
The correct thing to do, in such a circumstance, is to thank the person for his or her honesty, cut your losses, and buzz off. The incorrect thing is to get all huffy, deplore his or her "racism", and attempt to change his or her mind. For many--if not most--people, marriage, family and ethnicity are completely intertwined.
Also, there were a goodly number of genocides in the 20th century that made the survivors very committed to replenishing their numbers. In short, for some people, their very ethnic nation is their primary value, and when it comes to looking for a marriage partner, I think you should respect this. When a very decent and handsome Armenian chap told me twenty years ago (when I was still dumb enough to ask men out for coffee) he could only marry an Armenian, I got it. I got it because after a lifetime in Toronto, I had realised that other ethnic groups are much less likely than mine to "marry out."
But again, this all depends on core values. The biracial Catholic wife of a Catholic man from a separate ethnic group once contacted me because she was horrified that a Polish-Canadian in her Catholic mothers' group (or whatever it was) had stated that she would be very upset if her children married non-Poles. My friend-from-way-back had heard similar remarks from Eastern Europeans, and she was shocked such views existed in modern Toronto. (If my memory does not betray me, she married very young and thus was spared my adventures in multicultural dating.*)
Well, I wrote her an entire essay on the topic, which may have killed her, as she never got back to me. But at any rate, that Polish-Canadian mother will probably make a hellish mother-in-law unless her children all share "Poland" as a core value and marry only other people who have "Poland" as a core value. Chances are, though, that she may have given birth to someone who falls in love with Japanese culture, goes to Japan to "teach English", and comes back to Toronto only at Christmas with his Japanese wife and three beautiful Shinto-Catholic children. And good for him, say I.
If you can think of "shared core values" which absolutely would not work to foster a marriage (besides "open marriages", i.e. sexual promiscuity), do let me know. I was going to say consumerism, but if a couple are united in their love for the earn-to-spend lifestyle, it might not be fatal. I suppose it depends on their degree of honesty and ability to share. For example, if a man loves to collect Matchbox toys, he can't really kick up a fuss if his wife loves to collect shoes.
*Being told over a friendly lunch that my East Asian luncheon companion was attracted only to East Asian women of his own ethnicity felt rather old, though, as I was in my 30s and actually had no designs on the chap that I can recall. But what a relief to have understood at last that only men who are "into redheads" will ever be interested in little me. "Redhead" is not a core value, but it is a basic attraction thing, which seems to be way more important for visual men than for imaginative women.)