If women are more interested than men in a particular thing, that is a sex-based difference.
Or a gender-based difference, if women actually are more interested than men in the first place.
What does 'in the first place' mean?
This is a very equivocal line of reasoning. It's a sort of lazy, sci-fi "and then quantum mechanics shit happened or something" kind of explanation of things. Saying neurotransmitters are involved really says very little about sex differentiation in neurology and links to specific behaviours or outcomes in social matters.
I don't know what your objection is. Any differences in interests between men and women is a difference that must be a difference in brain states. I am interested in some things and not others because my brain responds more favourably to some things and not others.
Toni thinks mens and womens brains are different. Different brains produce different interests and intelligence and thoughts and personalities. Interests and intelligence and thoughts and personalities all influence career choice. Career choice influences wages and salaries. The gender pay gap is measured on wages and salaries.
I don't understand why you are having such difficulty with this line of reasoning. Do you reject one of the assumptions, such as that interests and intelligence influence career choice? Or do you agree that while there is a difference in brains between men and women, those differences all reside in areas that have no effect on career choice?