Salzman and Lawler: two moral degenerates trying to rationalize their perversion through twisted sophistry.
A wise old Jesuit friend explained that such sophistry is possible if you do not know what is normal or refuse to deal with it. Not common, but normal.
It is obvious without proof that sexuality is meant for procreation and that the parts of male and female are apt for that.
So normal sexual orientation is ordered toward the opposite sex. Therefore any other orientaion is disordered, including those of orientaion toward same sex, children and different species.
Why? Because they do not and cannot fufill the normal reasons for sex.
Not difficult if the authors were not trying to justify the not normal.