An overwhelming number of narrations clearly state that this part of the body of a man is a part of his Aurah and should not be exposed.

“The Aurah is between the navel and knee” (Kanzul Ummaal/Al Mu’jam Ul ‘Awsat)
“The thigh is part of the Aurah” (Bukhari)

Hence, gazing at any area of the Aurah of another man is prohibited and doing so is sinful.

Muslims are urged to heed this important command of the Shariah.