"Can a man take fire in his bosom and his clothes not be burned?"
Solomon is trying to tell young men especially that the urge they have to meet their sexual needs cannot ever be allowed to flow outside the boundaries of marriage. It is like taking a hot ember and putting it into your very heart.
Everything about you will change if you go outside of marriage and have an affair. Your affections and your focus – as well as your conscience and sensitivity – will be changed. Sexual unfaithfulness is a fire. It seems, at the beginning, that it is a fire that cannot be denied; but then it becomes a fire that cannot be quenched, burning up everything that is most precious to you.
I have watched what happens to men who want to add this "extra" to their lives. It is like adding a fire to the middle of your living room. It will not add just the little benefit that you think it will add. It will go way beyond where you think you can contain it.
Solomon is saying to think about what will happen, not just what you want. The problem with most adultery is that the people involved did not consider the wreckage that their infidelity would cause.
Yes, you will miss out on a few moments of stolen passion; but you will also miss out on a ton of misery, destruction, and pain.