I can't say for sure, but it is abnormal behavior shared by a very tiny percentage of the population. It is certainly not the norm.

"Mental illness refers to a wide range of mental health conditions — disorders that affect your mood, thinking and behavior"

Disorder-disrupt the systematic functioning or neat arrangement of.

So given the definition we can clearly see that homosexuality is a Mental Disorder. I come to this conclusion because in the definition is says disorder(definition above) which relates in this definition because being homosexual disrupts the systematic functioning of the reproductive system. additionally it is a disorder that effects your mood thinking and behavior for the same sex : )

http://www.mayoclinic.org/diseases-conditions/mental-illness/basics/definition/con-20033813

One of the biggest studies on the experiences of transgender people was the 2011 National Transgender Discrimination Survey (NTDS). It found that in the U.S., 41 percent of transgender and gender non-conforming people had attempted suicide, compared to a national average of just 4.6 percent.Jun 22, 2015

What is the reasoning for the above stats ?

Such a small percentage of people are trans, makes you happy hell do it. But intellectually believing your one gender than the one you were physically born with has to be something.