The total picture of strength, of confidence, of an assertive man with a powerful body and an untroubled mind. It is what you want to be, yourself - and that trips a lot of the "desire" switches. Not to be crass, but - there's a reason why nearly all straight porn features prolonged, almost loving close-ups of huge erect penises, quite often without even a woman in frame. It is a stand-in, an avatar, of the male viewer's desire of his own achieved embodiment.
THIS is something much more serious.... but it's the same principle at work.
"Don't think it hasn't been a little slice of Heaven just because it hasn't!" --Bugs Bunny