Easy to disprove.

1. who buys USA's milk? USA, largely. Doesn't export enough because doesn't pass standards in most other places.

2. Sex and gender diversity is proven both global and historical.

Therefor, something else is the motivator. Perhaps a drop in colonization-enforced repression? (Historical Europe had more diverse gender identities before the spread of Christianity ... and colonization.... no I don't pair them quite together, but they definitely travelled together)

Edit: don't forget that lactose tolerance is not a majority feature of humans.