This is a very interesting thread, it really gets to the bottom of a lot of TRP things. This argument is pure semantics -- but semantics are incredibly important. Mankind's quest for meaning is fundamentally a semantic quest, because semantics just means "meaning." So I agree with /u/PoopInMyBottom that this argument is about "semantics," but I'm also quite clear that semantics are important as fuck.
I do not agree 100% with anyone's definitions of Masculinity vs. Femininity here. (Though I think /u/CisWhiteMaelstrom's is the best definition presented so far.) I would like to illustrate my way of thinking about Masculinity vs. Femininity, which seems to be slightly different than his. I'm curious if you guys will agree or disagree with it.
What is a solid and what is a liquid?
I think the only way to answer these questions is to ask: what are the differences between these things? The only way these terms make sense is if they are defined in relation to one another. (If you google the definition of solid, it even says "a subject or object that is solid rather than liquid", because these terms only make sense in relation to one another.)
We know that "solid" matter is more firm and stable than "liquid" matter. Therefore, firmness and stability seem "solid" to me.
We know that "liquid" matter behaves differently than "solid" matter: it flows freely. Therefore, flowing freely seems "liquid" to me.
So what is masculinity, and what is femininity?
What is a male, and what is a female?
The only way I can answer this question is: WHAT IS THE DIFFERENCE BETWEEN THE TWO?
We know that males have penises and women have vaginas. Therefore, having a penis seems "masculine" to me, and having a vagina seems "feminine" to me.
We know that males have more muscle mass than female people. Therefore, having muscle mass seems "masculine" to me. We know that males tend to be more interested in engineering/scientific pursuits. Therefore, engineering and science seem "masculine" to me.
We know that females are more neotenous than male people. Therefore, looking young seems "feminine" to me. We also know that females are significantly more concerned with things like make-up products than males (at least judging by the size of the beauty-products market for women.) Thus, having a young-looking and heavily-made-up appearance seems "feminine" to me. (The Bieber picture is a perfect illustration of a feminine man, albeit an attractive one. He looks young and heavily-made-up: these are feminine traits.)
I think in order to define "Femininity" and "Masculinity", we gotta use the same semantic strategy that people use to define "Liquid" vs "Solid": We just have to look at the differences between the two and then define "Femininity" and "Masculinity" based on that.