Sex outside of marriage is a sin. But what about "less than sex"? Inspired by Is masturbation a sin?

This is a tough subject that I've grappled with my whole life.  It's clear in God's word that sex outside of marriage (adultery) is a sin.

But, what is acceptable if sex isn't?

More clearly, I expect that kissing isn't a sin.  But how far beyond kissing is allowed?  How far is "too far"?
This is a very difficult issue.  And for someone trapped in the chains of sexual sin, there is no easy answer - but there is the stunning grace of God for freedom.

I would answer this question by starting with another question: Do you want to live your life seeing how close you can get to sin without actually crossing the line?  Are you motivated to be the most Godly person you can be (growing daily in Christ-likeness), or are you motivated to "get away with" as much as possible without actually going to hell?

When Jesus said, "But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart" in Mat 5:28 he was addressing the inclination of the heart.  

In my opinion, when you indulge in sexual lust you are sinning.

We then have to define "sexual lust".  Generally, lust is any desire which elevates your needs over another's.  A good working definition for sexual lust might then be, any thought or action that seeks to use another human for your personal gratification without love or respect for their person.

So if you apply the spirit of this to your treatment of your girlfriend/boyfriend, it becomes "is doing X with this person bringing both of us closer to God, respecting them as a person and expressing pure Godly love?"  If not, I think you have your answer.