Good grief!

Sex and gender have been used interchangeably probably since about the time we stop using ughs and grunts to communicate and developed language. A very long time.

Around 2001, they began all these studies, NAS, IOS and other institutes and for clinical studies only, it was decided to:

In the study of human subjects, the term sex should be used as a classification, generally as male or female, according to the reproductive organs and functions that derive from the chromosomal complement [generally XX for female and XY for male].
In the study of human subjects, the term gender should be used to refer to a person's self-representation as male or female, or how that person is responded to by social institutions on the basis of the individual's gender presentation.

Point being if you are born biologically male or female, but you lean towards a different orientation and act on it, nobody cares! Leave our children alone though.