As a man thinks, so he is.

My post title is, I am sure, a Bible quote, but I can't find it.

.... Ah, there we go ... Proverbs 23:7 but only in older versions (KJV) so probably doubtful.

anyway, linking this to philipians 4:8, which advises us to think on constructive things, presumably if end think about bad things it is not good.

Jesus says that in looking at a woman lustfully we commit adultery.  So if you think about murder in a computer game (GTA) are we guilty of murder?

And, getting silly now, since a frequent male fantasy is about lesbians doing what they do, does that mean that the men are also lesbian?

This is clearly an over-literal interpretation.  The point is that we can't judge ourselves to be righteous purely on the basis of actions; it's also about the secret stuff we think is hidden away in the darkest corners of our minds.

Which in my case is pretty sordid and means that I am exceedingly grateful that I am under God's grace, and that through the blood of Christ am not eternally judged on the basis of my thoughts. 
But we can't rest with the status quo.  To believe in Jesus but continue in our sins is not faith - even demons do this.  The evidence of faith is repentance, turning away from sin to a new life.  Thus, Paul urges us to be transformed by the renewing of our minds.

So some of those thought patterns will have to go.  And I am not good at this, so would value prayers.  I am quite good at postponing mental sins, it's the permanent nature of the required abstinence that never seems to succeed.