The concern is that the law will enshrine the affirmation model of treatment in legislation. Which is to say, if a child says they are the opposite sex then the therapist must affirm that, and thus any therapy which fails to do that but instead seeks to explore the reasons around these feelings would be deemed to be conversion therapy because 'attempting to change a person's ... gender identity' is prohibited.
This is what is meant by the bill having 'unintended consequences' because it is muddy and badly drafted.
We wouldn't affirm the body dysmorphic condition of anorexia without looking at the underlying issues, so it makes no sense to affirm gender dysphoria and dish out medicines and even surgery without exploring deeper issues first.
In most children body dysphoria resolves itself by adulthood and they do not transition. To deny children exploratory therapy, which is done through qualified counsellors and therapists on the NHS, to look at both their body dysphoria and wider issues, is to set them on a pathway that may not be the best solution for them.
As far as I know, most same sex attraction doesn't disappear in adulthood, whereas gender/body dysphoria can and does. Deciding you're gay does not entail a pathway of puberty blockers and hormones, and sometimes surgery. Surely a cautious approach for children and young people is more appropriate than banning exploratory therapy.
There have already been cases where children have been put on a medical pathway to transition without any exploratory therapy and after only brief meetings with a doctor because the doctor was happy to follow the affirmation model without further evaluation.
Even the Katolinska Institute in Sweden, which pioneered much of the established gender treatment in young people, has stepped back from the affirmation model and ended the use of puberty blockers and cross sex hormones. France and Denmark have similarly begun to adopt a more cautious approach.