Do I have to stop my head from thinking about sex?

Suppressing isn't a good thing at all. Does more damage than it will do good.
Abstaining and building resistance to "something" is built by feeling and obviously not taking part in said activity, no matter what the "thing" is.

You can't force your mind to not think of something because the moment you think "i'm going to stop thinking about this" and then you literally think about it the moment your sentence finishes.

 

Well, yes you need to stop it. Once you get the temptation and the image stays in your head this often leads to urges and relapse. There are a few techniques you can use to avert thoughts, counting in your head repeatedly. Listening to the wave surfing audio files. Some people like to radically change whatever they were up to, startimg a physical actovoty etc. Either way, you keep using the methods till you can control them better. Also avoid things that stimulate these feelings. And this could be as simple as fb. So be true to yourself. It's an addiction and when it's still strong it tricks us a lot.

 

There is much power in your words and can alter the direction thought process in your mind.....

When such a thought crossed your mind, stop it, if you don't, It will breed into meditation over such a thought and finally it will be acted upon