Here's my take, for it's worth. Sin can happen in a relationship (sexually) one of two ways. The first way is the most obvious, and that is a sexual act that is sinful. In this case, they Bible only expressly forbids intercourse. The second way that sin enters in is by glorifying your partner or your relationship above God. That's where the grey area comes in. If God is not glorified in it, then I wouldn't recommend it. Anything that brings you closer to the opportunity to commit a sinful act is not glorifying God, and that is why I think it is important to maintain strong boundaries that keep you far from the line, rather than going right up to the line before you stop. Additionally, in my experience, in a long-term relationship, there WILL be times when you cross the boundary that you have set for your relationship. So the question is, can you cross your boundaries without doing anything you will regret? That is why my wife and I set the boundary at kissing before we were married. It was hard, but it was well worth it. I have no doubt in my mind that we would not have made it to our wedding night otherwise.