What it means is the designation species often refers to what on closer examination actually is a sub-species.
Thats fine but I still don't see what this has to do with interracial relationships.
If we believe there are "interracial" relationships it must follow that there are different categories of humankind which are commonly referred to as "races." I reject that term because of the glaring semantic conflict which holds that there are five recognized "races," all of which belong to the human "race." That makes no sense. And the discrepancy cannot be dismissed by simply saying there is no such thing as racial distinctions.
While I will use the word "race" in informal conversation I reject it in discussions such as this because I believe the word and term species and sub-species is more accurate -- in spite of the fact that many people resent being informed that humans are in fact a species of animal. And like most other species there are readily recognizable variations within our species which may be referred to as sub-species. It doesn't mean that one sub-species is generally superior or inferior to another but is simply a means of referring to recognizably different characteristics..