Dr. Anthony Flood explains in a very simple fashion why marriage across religions, race, culture, or even class is not a good idea;
“Aquinas’s principle states that the greater the similitude, the greater the stability and permanency of the love. The greater the permanency of the love, the greater the real union the lovers seek and likely obtain. Thus, a greater similitude prima facie affects a greater real union between friends. The ultimate rationale for this principle draws from the relation between unity and union. As unity is the principle of union, the closer two things are naturally, the closer they can be through love. At the level of persons, the closer a relationship with another person comes to unity the stronger it will be. Since self-love arises immediately from substantial union, we can say that the more of a lover’s love for the beloved approaches one’s own self-love (and vice versa), the greater the loving union between them will be. In other words, the most stable and permanent love relationship will be the one that approximates most each person’s love of self. “
The Metaphysical Foundations of Love — p. 35
Kinists do not discourage marriages across racial lines because they hate people. They discourage marriages across racial lines because they love people. They know the lack of racial similitude between bride and groom will likely be a future friction point and so fracturing point of the marriage. The goal of entering into marriage is for each partner to have a partner who is in a multitude of respects a reflection of themselves. The more harmony of interests between the one male and one female marriage candidates the more likely that the marriage will be a success. Introducing vast differences in race, or class, or religion, or culture, or values as in each party coming to the marriage altar is a sure-fire means to guaranteeing that the marriage will fail or at the very least not be as happy as it otherwise would have been if those differences had been taken into account before the marriage was entered via covenant.
Now let me anticipate an objection to the quote as coming from earnest Christians. I can hear many protesting at Dr. Flood’s recommendation of the necessity of self-love. I will only say here that if you want a guarantee that a marriage will fail marry somebody who doesn’t have a healthy and Christian love of themselves. When Dr. Flood talks about “self-love” he is not talking about narcissism. He is merely saying by way of metaphor that people who are comfortable in their own skin should marry people who would likewise be comfortable in their skin if they had to be.
Doubtless, there are inter-racial marriages that work, as well as marriages across class lines and culture lines and religious lines that work. Praise be to God for those marriages. However, I’ve also seen healthy three-legged dogs who run well, but that doesn’t mean that we should promote either inter-racial, inter-religious, or inter-class marriages or the cutting off of the legs of dogs. The fact that something odd or unusual works isn’t a recommendation for continuing to pursue the odd and unusual.