Dr McHugh has good reason for his beliefs, the truth is there should be no argument that transsexualism is a mental disorder as it is defined as such in the Psychiatric DSM-IV ( Diagnostic and Statistical Manual of Mental Disorders, 5th Edition: DSM-5). 302.85 Gender Identity Disorder in Adolescents or Adults
Gender Incongruence (in Adolescents or Adults)  A marked incongruence between one’s experienced/expressed gender and assigned gender, of at least 6 months duration, as manifested by 2 or more of the following indicators: [2, 3, 4]
1. a marked incongruence between one’s experienced/expressed gender and primary and/or secondary sex characteristics (or, in young adolescents, the anticipated secondary sex characteristics) [13, 16]
2. a strong desire to be rid of one’s primary and/or secondary sex characteristics because of a marked incongruence with one’s experienced/expressed gender (or, in young adolescents, a desire to prevent the development of the anticipated secondary sex characteristics) 
3. a strong desire for the primary and/or secondary sex characteristics of the other gender
4. a strong desire to be of the other gender (or some alternative gender different from one’s assigned gender)
5. a strong desire to be treated as the other gender (or some alternative gender different from one’s assigned gender)
6. a strong conviction that one has the typical feelings and reactions of the other gender (or some alternative gender different from one’s assigned gender)
The sad part of this is the real victims of this push to make transgenderism main stream are the young boys who's parents are pushing them into pushing transgenderism. It is well known that children tend to be gender neutral, ie boys dressing up like mom. However they normally out grow this stage spontaneously if left alone, and it is child abuse to promote anything otherwise. What is also just under the surface is how patriarchal transgenderism is. The primary victims are women and young girls. These is the gender that is being put upon, forcing them to endure sharing a locker room and showering with men with men genitalia; something that is not only unwelcome but can be dangerous if these man are predators.
Let is also be known that there is no country that allows such things. Yes, there are unisex bathrooms but they are not the norm and have a warning displayed. Further the transsexual's have also stated they do not want separate bathrooms, they want to use what ever gender bathroom they identify with; they do not want accommodation they want full acceptance. It should also be noted that this has nothing in common with homosexuality, where gays and lesbians have no desire to share bathroom and locker rooms with the opposite sex; as Dr McHgh said "Claiming that this is civil-rights matter and encouraging surgical intervention is in reality to collaborate with and promote a mental disorder."