God say in the Quran : « Successful indeed are the believers, Who are humble in their prayers, And who keep aloof from what is vain, And who are givers of poor-rate, And who guard their private...
Some scientist said : Because you not allow to enjoy your private part except with your wife "for man" or with your husband "for woman" . and their prove is from The Quran chapter 23 "mo'menon" verse...