Adultery is always stupid. In fact adultery here is being called dumber than stealing. We are offered a comparison between these two sins - and in the end, adultery is considered the worse of the two.
This passage is interesting, because although it does compare stealing and adultery, it does not condone either. The comparison is to how the theif is viewed vs. the view of the adulterer. The thief is actually shown compassion, especially if his stealing is due to being hungry. The passage tells us that a thief is not despised if he steals to satisfy his own hunger. We all understand hunger and the drive to satisfy our appetite when we have not eaten in a while. The thing about this proverb though is that after saying this - we are brought back to justice. If caught though, the thief will have to repay sevenfold for what he has stolen. This is the case even if the thief has to given everything in his house to pay that debt. There is mercy toward his situation - but not mercy toward the actions he took to remedy it.
But the adulterer is by default NOT given the same grace. We all probably know lust as well - but to actually go out and take another man's wife to satisfy it is wickedness. The adultery is also guilty of stealing - stealing the sanctity of another man's marriage. He is stealing another man's wife - taking her affections - and taking from him the vows that were made to him in the sight of God. This is not viewed with a gracious attitude here. Whereas a thief is not despised for his actions to alleviate his hunger - the adultery IS despised for taking another man's wife. If justice falls on the hungry man for his stolen food, how much more will God's justice fall on the adultery for stealing the sacredness of marriage and a home? There is going to be a cost - and that cost is high.
Ask David and Bathsheba what that cost involves. They will tell you that the cost far outweighs the pleasure of the moment. Ask David's family who also paid a high price for the attitude toward marriage and sexuality that was unfortunately passed in that family? Ask Samson if it was worthy his two eyes to commit sexual sin and satisfy his lusts in ungodly ways? Ask the people of Sodom and Gomorrah whether it was worth it to despise God's plan for marriage and go their own way? These are all examples of those who had to pay - and some pay with the very substance of their house. Adultery costs - and to think that we can get around that cost is ignorant. That is why dear saints, "Adultery is always stupid."