Billy Birch is a guy whose blog I've been following for some time, and someone who used to be a member of SEA. For those of us on the theological blogsphere, he is very well known. It is also well known that he committed a terrible sin a few months ago, shortly after he had left SEA in fact.
Well now he has a new blog: White Picket Fences. On it he talks about what he had done and why. I think it best if you read his own account. Personally, I find it very encouraging how much forgiveness he has recieved, even the one he hurt.
One important thing is that he idenifies the fundamental reason why he sinned: he kept his temptations a secret. This is incredibly powerful. In his book Celebration of Discipline, Richard Foster identifies confession is one of the fundamental disciplines of Christian life. God designed the Christian walk to function as a community where we hold each other up, and defend each other. This includes defending each other from our own sins.
If you are struggling with a sin in your life, don't make Billy's mistake: confess it. Have your brothers and sisters in the Lord hold you accountable for what you do, and what you think. It is the most powerful way to stave off sin.