There's plenty of study on relationships between people of opposite sex. I don't even know how you could say this.
And what subjects would those be?
Eugenics, heavier-than-air flying machines, and the luminiferous aether.
Edit: I was trying to be snarky, but I just realized that each of those has been thoroughly studied at one point or another. Just goes to prove your point, I guess, heh.
The formal scientific study of certain subjects is not permitted in our society, so whenever anyone attempts to study them anyway without official sanction, or simply compiles honest observations, they will be smeared like this.