The Antisocial Socialist

Here's how I figure it. Most products, and almost all beauty-related products, must be divided into Products for Girls and Products for Boys. It is marginally okay to dress a girl baby in a blue onesie, but it is never okay to dress a boy baby in a pink onesie. The gender division continues into adulthood, when Products for Girls usually cost twice as much and come in pink.

There's no reason, as I confirmed with an experiment on a friend's hair, why men can't use women's hair dye. There are more colours, more varieties, and it's cheaper. Therefore, the only reason men would buy men's hair dye is that they feel that their masculinity would be somehow in doubt if they bought women's hair dye. Hence, patriarchy. Though I like marrythebed's theory better.