Gender is a construct. Repeat: Gender is a construct.
What does that mean. exactly?
Gender, as opposed to sex, is an idea created by society. Sex is biological. The only inherent difference between men and women is the ability to produce children. Period. Everything else that you think of as male or female is an idea created by society.
Women are not naturally more nurturing, men are not naturally more aggressive. Women were not just gatherers, men were not just hunters. Not all men feel the need to sow wild oats, not all women fall in love with every guy they sleep with. These are ideas created by society to keep the basic power structures intact, regardless of whether those power structures are fair or even the most efficient way of running a society.
From the moment we know the sex of a child, we start putting gender roles on them. Tiny babies who wouldn't know blue from pink get dressed to match their sex. Guess the wrong sex of a non-distinct baby in a gender neutral color like green and the mother will issue a sharp correction pdq. The sex of a baby shouldn't even matter, since it will be a good long time before that baby is having any sex of it's own.
Everything that we think of as feminine or masculine is created, and if it's created rather than inherent- it's changeable. Men can love babies and fashion, women can love football and fast cars . Liking things that don't "belong" to your gender doesn't make you a Tom boy or a Girly man, it just makes you a human with varied tastes and experiences.