Couples who tolerate infidelity do not live happier lives together. They live lives where they can never trust each other and know that they are not the only person in the world for them. I think women especially like to believe in fairy tales even if they are not true.
What works for some marriages may not work for others. It really depends on the individuals involved and can not be caught under a blanket statement like an open marriage leads to unhappiness. A married couple needs to decide what type of relationship they want to have. Do they want to be faithful only to each other, or do they want to be committed but still occasionally enjoy some non-marital carnal lusts. In a country where over half of marriages lead to divorce anyways, it really isn't fair to say that one marriage style or another leads to more happiness when so many lead to failure as it is.
Couples who tolerate infidelity do not live happier lives together. Infidelity causes a wide range of emotions such as anger, jealousy, and causes mistrust in a relationship. Couples who commit acts of infidelity towards one another are more likely to split up than to stay together. It is difficult to tolerate infidelity.
I do not believe couples who simply tolerate infidelity live happier lives together. I think some people are in open relationships and even marriages, but that goes far beyond tolerating infidelity, they seek it out. Those that tolerate infidelity are more than likely not happy in the situation but do not want to leave it at the same time.