If you put a male and a female baby next to each other, you wouldn't be able to tell which was male and which was female until you removed their diapers. This is because they haven't learned what masculinity and femininity are yet, and modeled their behaviors accordingly.
This is what is meant by gender being a social construct - what is deemed masculine and what is deemed feminine are defined by the society rather than biology. This has nothing at all to do with biological parts, or even with sexual orientation.
And you are correct, masculine and feminine are very real, and being one or the other is plenty healthy. There is absolutely nothing wrong with being a male, cis-gender, heterosexual. If that's what you are, no one should ever pressure you to be anything else.
As I say very often, this area of study is brand new, relatively speaking. We're still learning, so take everything with a grain of salt, but also DBAA.