Thoughts lead to Action; action leads to Habit; habit to Character; and character to Destiny.  If you want to write a great destiny for yourself, your thoughts have to be right.  To stay away from wrong thoughts, and have only right thoughts you need to know the source of thoughts.  Why do you think what you think?  What brings wrong thoughts to you instead of right or vice-versa?  The answer is, whatever we experience through our five senses that influences our thoughts.  What type of people are you surrounded by; what type of company you keep?  What type of programs you watch, what kind of music you listen to?  All that influence your thoughts.  Hence, if you watch indecent stuff, so will be your thoughts, indecent and wrong!  Wrong thought will make you take wrong action, develop wrong habits, then wrong character, and finally unfortunate destiny.