Mutants are defined by a single gene which they have and humans do not.
Luna was (in)human.
She did not have the gene.
What I am proposing is that there can (biologically) be no such thing as a mutant Inhuman hybrid because Inhuman genes dominate Mutant genes to the point of dissolution, just like there can be no such thing as an Inhuman/Human hybrid either.
Sabertooth's completely Human son.
Was Sabertooth shooting blanks, or did he unwittingly breed with an Inhuman who did not know that she was an inhuman? Thus stripping the mutant gene from bis child.