"Or can a man walk on hot coals and his feet not be scorched?"
This verse is the second verse dealing with fire in regards to adultery. The previous verse compares being involved with an adulteress to taking hot coals inside one's clothing or near one's bosom. This verse compares being involved with an adulteress to a person who would walk over hot coals.
The two pictures deal with the passions that an adulterous relationship excites. I have worked with a number of people involved in this sin, and they have testified to the all-consuming nature of the passion in this type of relationship – for a little while. It is like an out-of-control fire threatening to consume one's heart and become the preoccupation of one's whole day.
If it is more than just a one-night stand, an adulterous relationship is white-hot with passion for between two weeks and six months and then it cools. The problem is that many of the decisions that this "couple" make are during the white-hot fire of passion. They think that it will stay this passionate; it will not. They have never felt anything like this before and want it to continue, but it will not. It will consume their emotional energy, financial resources, and other relationships and then die out only when all the fuel is gone.
One of the things that Solomon is saying here in this verse is that committing adultery is full of passion, but it is like throwing hot coals on the path that you must walk every day. It will scorch you. If you give in to this immoral passion, all the rest of your life will be burned along with your soul. One does not just add adultery to the mix of one's life. It comes in to take over; it spreads itself in every area of life. You cannot keep this kind of passion contained in a little pot in the closet.
Solomon is saying that adultery will burn your heart; it will burn your whole life. Don't go near it. You can't control it. You can't contain it. It will scar you in ways that you can't imagine, so avoid it just the same way that you would avoid a walkway that was strewn with hot coals.