My understanding of masculinity is that it is the characteristics by which other men rate comparable level of attraction from females. Men often think big muscles brings all the ladies to the yard, so that is one possible display of masculinity, leaving “weaklings” to feel inadequate.
My impression is that people think showing off boners in public scares away the females, so it does not seem like a good candidate for being a display of masculinity. But if we assume that showing off boners is something men believe woos the women, is the aforementioned difference casually noticeable? Science can reveal things that nobody would ever notice in day to day life.