It is, though.
When you seek out a wife/sexual partner, well-adjusted, normal people will compare prospective partners to their mother in the case of men, and too their father in the case of women. For any given female she desires a sexual partner equal to or better than her own father, not someone wholly dissimilar to him.
When a female or male thus prefers other races, this is caused by a disturbed parent-child relationship. Therefore those who have interracial relationships come from a bad household, are socioeconomically challenged, and are therefore more likely to have a lower IQ than average, and are more mentally ill.
Secondly, the amount of empathy you feel towards a person is a function of genetic similarity, the more dissimilar you are from another person, the less empathy you will feel for him. This means that, in an interracial relationship, family members are less affectionate towards each other compared to a normal family.
This is why interracial relationships are more likely to result in domestic violence or divorce, no matter which races intermingle.