People dislike interracial relationships for various reasons. Some people are purely racist while others frown upon it because it may cause confusion among the children and future generations.
Children become confused about where they belong, in which racial group specifically. In future generations, the interracial relationship which created the grandchildren can surface again. For instance if your grandparents were an interracial couple, you may be black, and have relations with a black person, but your child may come out very light skinned because of this previous interracial relationship.
Another factor which could cause especially older generations to frown upon interracial couples relates to religion and traditions of a certain culture.
It may be that this person is afraid that the traditions will not be upheld in a mixed family.
For more information on this, take a look at this article.
