Why have relationships between men and women altered since the 1950's and 1960's?

Sex refers to the biological aspect of difference in the anatomy. For example, male and female. However, gender refers to the social differences based on the above biological difference. For example, woman, man, transgender and so on. 

The socialization has proven to be crucial in internalizing the roles attached to gender. This is called gender socialization or socialization of gender roles. For example, cooking is woman's responsibility and earning an income is that of a man's.