I really think the western aspect of masculinity focuses upon self determination and will. So a real man is one who understands what they want and adapts themselves to pursue it relentlessly with the understanding that the effort they put in is worth it.
XY Chromosomes, the rest of it is just window dressing and other peoples desires. One of those Roman guys had a good point with "stop arguing on what a real man is and be one"
Since this is explicitly opinion:
Discipline, self-effacement and a willingness to sacrifice for others. Doing good for the sake of doing good, virtue as its own reward.
Real men are secure in their masculinity and don't feel the need to constantly prove they are men. It should feel as natural as breathing. Insecure men showcase very feminine behavior they may not even be aware that they are doing. A sense of duty, sacrifice, and motivation.