1) Homosexuality is not a mental illness. There's nothing to "treat." Trying to "cure" it is like allowing a company company that sells essential oils claim that their products cure cancer. They can believe it all they want, but it is a monstrously irresponsible falsehood.
2) It has been thoroughly documented that conversion therapy does not work, and can instead cause much more harm in the process.
3) Individuals subjected to conversion therapy were very often forced to do so against their wills, particularly those who were underage.
4) Banning conversion therapy (which, again, does not work) does not prevent an individual from seeking professional counseling over issues of their sexuality. Medical professionals are ethically obligated to "do no harm" in treating their patients, and conversion therapy is inherently harmful.