Easy answer.
When you see them on television in those tight hugging sports clothes, they've already worn those same clothes at least a 1000 times or more. It takes four to eight years in order to reach Olympic levels, if not longer.
By the time all the cameras are pointed at them, they'll have become so accustomed to everyone looking at their junk, that they won't care that the audience has increased a thousandfold, as it's a non-factor in their mind.
And athleticism itself changes how a guy deals with nudity. They're forced in so many situations with communal nudity, fast clothing changes and almost-nude physical examinations that they won't care after a period of time.
Their nude form is just something that's part of life. And if some people enjoy the sight and become sexually interested, it's just a fringe benefit of their way of life.