That's a creative curse. Works pretty well. Basically a gene driver.
I note that for human males to be viable for eluding the curse , there should be no such thing as a half-demihuman.
Every demihuman birth leads to a full demihuman regardless of father.
That's more or less feasible if we're assuming a separate chromosome with all the demihuman genes on it. Like, it's a modified X chromosome or something.