These are interesting questions that I myself love to delve deep into. I am, you could say, almost worshiping traditional masculinity. I find it so fascinating and inspiring, and thus of course extremely attractive. That has really pushed me to become my own ideal, demeanor wise. I guess there is a risk that it becomes an "act", but then so what if you're really into it and feel comfortable in your own skin thanks to it? Not that I was at all effeminate before, on the contrary I was quite the "average joe" masculine type. But since my fascination dawned on me, it did also change me in very fundamental ways, how I walk and talk, what expressions I use in daily speech. I also embrace my male attributes like growing a beard, not caring about where I have body hair etc.
Many times, especially in the homo community, traditionally masculine men, or "straight-acting" men are often ridiculed and even accused of pretending to be something they're not. Their demeanors are often dismissed as social constructs, and that they are slaves to the social demands of society.
For being so demanding of tolerance and equal rights, the gay community sure is one internally intolerant minority. So much that in order to fit in, you need to be a certain way. Thus, I've also seen regular guys spend less than 2 weeks in the mainstream gay community before turning into lisping queens, and at least to me, every bit of manly strength and virility gone. And all I see is a shell who conforms to what is expected of him, so that he can fit in.
Some types of behavior are no doubt social constructs - effeminate, stereotypical "gay" behavior, especially so!
But no matter how much some try to deny the fundamental differences between male and female, they're there nonetheless. As men, our (generally) bulkier build make us naturally swagger when we walk, our deep voices makes us talk a certain way. The sound of our own voices resound in our heads, and I believe this affects our thought patterns as well. Or course the vice versa could be said for women and femininity.
Of course, what makes a man depends on who you ask. For example, I've gone on 1st dates with guys who describe themselves as masculine because they can change the oil on their cars, still act like bitchy queens. I've had dates with guys that no doubt lie about their demeanor only to have the purse fall out of their mouths after the first 10 minutes.
I may not be able to provide an answer that is satisfactory to everyone, what manliness is,... but I definitely know what it's not.