Why are the male sex organs held outside body?

Incorrect premise: The premise of the question isn't accurate. While the testes (primary male sex organs) are located outside the body, the other male sex organs are not. Both the internal genitalia (epididymis, vas deferens, seminal vesicles, prostate gland, Cowper's gland, bulbourethral gland) and the external genitalia (penis, scrotum) lie within the body but protrude or extend outside in varying degrees during physical activities such as sexual arousal and micturition.