If a man as he grows up discovers that he has no (sexual) attraction to the opposite sex, and in fact may be appalled by the idea, it is understandable that he may seek medical assistance to determine if his situation is treatable. Why? Because the man “knows” it is ‘normal’ for a man to experience attraction to the opposite sex - it is the natural path that promotes the survival of the species (through passing on of genes), and his body is equipped with the necessary organs and capacity to give effect to that end. An incapacity or strong aversion to what is inherent to our nature is not ‘normal’.
Is the person physically or mentally ill? The former is testable (within limits) in various ways. The latter (in general) is judged against various criteria, including whether the individual’s capacity to operate in society is seriously diminished and whether the individual is distressed by his situation.
Now if a man experiences both the above lack of interest or aversion to sexual relationship with women but also a (sexual) attraction to men, do we now say oh, all is well? The man who is not distressed by this situation is certainly not regarded as mentally ill given the criteria used to judge mental illness. But the disinterest in the kinds of relationships which are the means to reproduce remains and the incongruence between bodily capacity and the behaviours to which one is inclined remains.
In respect of homosexuality the cause is not known and there is no generally accepted treatment, the individual may not be distressed (though some are) and is not rendered unable to function in the community, and so as a pragmatic matter no finding of mental illness is possible. But this does not change the observation that something is amiss.
