Its hardly about keeping a pure progeny. No progeny is pure in this day and age.
Its NOT about being superior to anyone, and its NOT about racism. There is perhaps a small minority who actually thinks along those lines. However, majority of the people dont. The primary reason for thinking along these lines, as I mentioned above, is to ensure a smooth transition for the bride and groom.
What you cook, what you wear, language you speak, customs and traditions you follow, they all differ from caste to caste. And while no food, dress or tradition is better than the next, its true that a bride and groom will have an easier time adjusting to each other, if some of these every day issues are pre-resolved.
But then, there are people for whom the above issues dont matter. Good for them. My point is, just because a person chooses to find an person from within his/her own caste, thats no reason to raise the racism alarm, because usually, its not.