I'm amazed at how fast the Christians jumped at this..
They're REAL good at not thinking about sex.
*cough* Ted hagard *cough*, catholic priests*cough*
There's no "solution" to not thinking about sex, nor should be.
I'm surprised that nobody has commented on the real danger of your post in that your post wasn't a fear of "thinking about sex", it's about thinking about sex with "other people", when you are in a relationship. Firstly this isn't ask aunt sue for relationship advice, it's a *conspiracy* forum. Haha.

The real issue is that you're likely to act on these thoughts and fantasies..
And there's no 'cure' for it.. what you should do is look for the reason that you're so obsessed with sleeping with people other than who you're with. It is either completely natural, or completely unnatural. The meaning behind it though, is up to. If you trust yourself enough to not act on your thoughts, and think you can remain faithful, then remain in hour relationship, and talk about it with your significant other. Otherwise, you need to remove yourself from the relationship in order to spare your boyfriend from what, at this point, is inevitable. By the looks of it, you are likely to cheat and become unfaithful, so I suggest you either come to terms with who you are as a person and what you're capable of, and make a decision now. Coming from someone who's been cheated on, yet never cheated, There is nothing worse - than knowing the person you care about most in the world thinks you aren't enough, or feels you are inadequate. You have an obligation to your boyfriend to make a decision whether to remain in a relationship, or become the whore you envision in your daydreams. Of may reflect on a dead spark between you, or it may be that you've been interacting with new males, or sexual interests in your life, figure it out, and spare your boyfriend the pain of realising his girlfriend couldn't spare him the hurt, and indignity of watching the person he holds the highest, rip his heart out. Don't play with people. Hurry up and make a decision. To be honest your casualness about the situation scares me, and the fact you are worried you might cheat befriended confronting the situation sickens me.

Yours truly,

Novel.