If anything, pink was initially considered masculine, because it was a diminutive of red, which was associated with war and power. That's why, as a paler shade of red, pink was widely used for boys' clothing.

By the late 19th century, however, with the popularity of Freud's and other psychologists' theories of infant development, parents were encouraged to differentiate their children's gender from an early age.

While variance continued for some time, with some parents favoring pink for girls and blue for boys and others doing the opposite, by the 1950s pink became strongly associated with femininity. Clothing manufacturers were also happy to promote this idea: if parents had a girl and then a boy, they couldn't hand down the pink clothes and would have to buy a whole new wardrobe for the boy. 

The feminist movement in the 60s and 70s added to the perception of pink as a feminine color by tying pink to traditional girliness. 

As a result, the unisex trend caught on with liberated mothers dressing their daughters in neutral colors. 

As with all trends, gender marking colors replaced neutral ones again by the end of 1980s. The resurgence of pink clothing for girls and blue for boys is partly explained by the fact that parents, who were brought up in unisex clothes in the previous decade, now wanted strongly gender coded clothing for their own kids. 

Fast forward to 2016, pink becomes androgynous with the so called 'millennial pink' showing up everywhere, which is not actually a color, but rather a trend for a range of colors from pale pink to salmon.