The very formulation of this question presupposes that it is already acceptable to have a mistress since it is only a question of knowing the circumstances. However, it is unacceptable to have a mistress in any situation.

A couple who have committed themselves to each other out of love must refrain from this logic which consists in the fact that when something is wrong in the couple, one of them systematically seeks a little comfort in the arms of another or another.

A penknife in the contract would only make the situation worse and would bring anguish and sadness to the person who is the victim.

No, I'm sorry, but there are a lot of ways to solve a relationship problem, like first talking about what's wrong, trying to make an effort on both sides so that the climate calms down and everyone thinks about what motivates them to stay together. If everyone sticks to their positions and makes no effort to communicate and this solution does not work, then you should meet a couple of counselors who could dig into the problem, who would allow communication between the couple and ask the right questions.

Even if all the solutions have been considered without giving any result, then we will have to realize that this story is over and take the difficult but courageous and saving decision to separate and turn a new page elsewhere.

Living with your spouse for a very long time can be a good thing in itself:
Building a family… InA human and real estate home Living with one's wife or husband requires sociability, daily tolerance. You can get tired without leaving your home already built. You may not get along sexually with your partner... you can entrust a third person with your doubts, your personal distress that your other half no longer hears, out of habit, weariness. You can have a misbehavior once or twice in your life as a couple... We can improve in bed thanks to an outside person who restimulates us sexually... After childbirth, women are disgusted or simply more stimulated by sexuality. Or a man can become impotent, lymphatic in the sex thing I know a man who only does this once a month. His wife would like to do it 5 to 6 times a week, if not more… Conversely a man who masturbates 3 times a day so as not to cheat on his wife and this wife, a virgin up to 25 years old, who does not care at all his own sexuality. What are they doing together? They can have a family life, as a couple, without common sexuality, and he can go and frolic in order to fulfill his physiological needs... which he now, with the wife's agreement, incidentally! Must say that she was tired of seeing her husband masturbating in bed, evenings and mornings. They didn't want to make separate rooms. Do not tell me that this completely normal man according to his entourage, is in an addiction! He just has a proper hormonal system to ''fuck anything that moves''…could. rescue his wife does not move. In fact many things can allow us to find somewhere else, once, often, the soul mate who would be useful to us for the stability of our couple. Everything depends on the spouse's acceptance of this breach of the promise, explicit or not, to stay together forever…. Sexuality is a normal thing that frustrated, prudish, uptight, austere souls would like to moralize, smear, minimize. It's more than a return to the 19th century!

It may depend on one situation to another as well as from person to person because a number of people prefer to have open relationships (in which the other person doesn't mind if you explore other options or have sexual relationships). In such a relationship, I don't think having a mistress is a point of concern. 
Other than this, one may have a mistress if he isn't committed to somebody in a relationship or marriage. If your partner isn't okay with it, I don't think there is going to be any circumstance when she would be. Had my man asked me this, I wouldn't approve of it at all. Instead, I would be concerned as to why he felt the need.