I think the meaningful harm being done by influencing your son or daughter to marry within their race is that it: (1) it's a horrible ultimatum to put on your son/daughter and (2) it promotes the arbitrary division and animosity among races which can manifest in all sorts of bad outcomes (differences in status, violence, etc.). That being the case, I bet we agree that division among races disproportionately affects minorities (as compared with whites), as you implied with your explanation re. status.
so, if either 1) your child has no desire to marry a person of a different race, or 2) the parent only expresses a preference but makes clear that the child is free to do as they wish, no harm is caused?
i would actually would argue that it (a person against offspring marrying a person of a different race)
usually had no ill effects on either the offspring or society as a whole, but may express, or make concrete, thoughts or opinions that are deleterious to society in some meta sense (even if they do not inflict damage on an individual level).
in fact i think one could generalize what i was arguing yesterday about group identity impacting how objectionable the attitude is into something like: it is less objectionable for someone to oppose marriage heterogeneity due to a desire to perpetuate in their own lineage a shared group identity than it is for someone to oppose marriage heterogeneity due to a belief that members of some other group are substandard partners.
that would hold true for members of a dominant or majority group as well as for members of small, persecuted or highly divergent minority groups, but i suspect, and it would be reasonable to assume, that the former motive is far more prevalent among minority with strong group identities.