I'm slightly confused. Do you mean if you get to second base then there is no way of going back to first so to speak?
While it is true that once you have sex you would find it hard to going back to just holding hands and kissing, this does not mean that once you reach a base you have to keep doing it. But I would think the reason for this is because of natural curiosity and the fact that you might rather like it rather than additction.
There are people who claim to be sex addicts there but as with most addicts of any kind there are usaully deeper, emotional/psychological reasons at the root of the problem.
I hope that helps.