As is often the case with pieces like this, they get a bit confused about the terms sex and gender making self contradictory statements like this:
... many believe that gender can be fluid, but that sex (i.e. biology) is fixed and binary. The perception of a fixed concept of sex/gender is challenged by trans people who feel their biological and gender attributes do not correspond.
If gender can be fluid, then it is not a fixed concept, and there is no need for it to 'correspond' to a sex.
While most children are likely to experience their ‘gender identity’ as consistent with their male or female appearance, some children and young people will disrupt binary expectations by dressing and behaving in ways usually associated with the opposite gender. Our experiences as researchers suggest that this kind of play is ‘normal’ among children who will grow up to be lesbian, gay, bisexual or heterosexual.
OK, so gender non-conformity is 'normal' for all children. But wait...
However, a child growing up believing themselves to be gender variant is likely to experience considerable distress, disturbed behaviour, withdrawal and potentially mental ill-health, if their feelings about their body are not recognised or acknowledged by the adults around them.
What is gender variant? Is that just the gender nonconformity mentioned in the previous paragraph?
This is, crucially, different from explorations of sexual orientation, which refer to the person’s sexual attraction to someone of the same gender.
Same gender or same sex? They made a distinction earlier, and now they're conflating the two.