In 1925, Jacob Goloumb, founder of the 'Everlast' (sports equipment) designed a special loose-fitting leather short to be used by professional boxers. These shorts allowed greater leg movement, which if you are a boxer --you'd want to 'float like a butterfly;sting like a bee'-right?
Random studies indicated that the restrictive tightie-whities may create excessive high temperatures and restricted movement which in effect could cause lowering of sperm count. Uh-oh.
Further, there was some indication that consistent and undue genital pressure may have had something to do with testicular cancer risk. Though all of this remains unsubstantiated and simply theory, it was enough to spawn a new age for the "boxer!"