The scientific evidence regarding conversion therapies suggests that these are extremely unlikely to change an individual’s sexual orientation, but instead are far more likely to increase levels of harm and distress, by directly implying that same sex attractions are disordered.
‘Free speech’ has never been accepted as a rationale to allow harmful or ineffective medical practice. Nor has it been used to allow providers to make unjustifiable claims about their practice (i.e., claiming that they can change sexual orientation in the absence of evidence that this is a reasonable possibility).
For consenting adults, allowing conversion therapy provides a degree of freedom, but only at the cost of encouraging a practice that is expensive and almost certainly ineffective at best and borderline fraudulent at worst.
For adolescents, allowing conversion therapy is far more likely to be quite harmful, as it stigmatizes an adolescent for their innate sexual orientation at a vulnerable time in life. It is far less likely to be used by consenting individuals in this case, and far more likely to be used as a result of parental panic or intolerance. The results of allowing this discredited practice to continue are likely to be very significant harm to the adolescent at a critical period in their development.