Taxonomy is a frickle thing.
Classification (and the various systems behind it) is a hot topic in biology and ecology.
Sex is not a necessary prerequisite (and is even a bad factor) to differentiate between species and sub-species.
There are (sub-)species who cannot intermingle even though they are classified as "equal" (monotypic vs polytypic).
Rationality would imply further discernment based on "Race".
Humans are Polytypic as the different races can intermingle and produce (fertile) offspring.
The studies you speak of are generally kept behind closed doors. Too much political sensitivity and stupidity from the general public trying to invalidate or ignore hard facts.
Ut vivat, crescat et floreat