The loosing of restrictions outside of marriage might help the institution as a whole, argues Christopher Ryan in his Big Think interview. When our culture responds negatively to natural urges, like seeking sexual satisfaction outside marriage, the results can do more harm to marriages than good:
And one more for the revisionist thinking about the marriage institution in this Big Think interview entitled “Income Inequality Helping to Build ‘Generation Single” with Chris Ryan, author of Sex at Dawn: How We Mate, Why We Stray, and What it Means for Modern Relationships. His words are excerpted above, but the three minute response in the video portion is worth a listen as he radically asks, “Whose business is it if a couple decides they’re going to allow a little casual sexual behavior on the side…it lets the pressure off.” He maintains that marriage has loftier aims and satisfies larger needs like child rearing, sharing a life and getting old with someone. The reality of who we are biologically–titillated erotically–and the expectations of lifelong fidelity, he says, are at odds and marriage expectations need to be changed to reflect the reality rather than “shoehorn” lives into the mold of a marriage concept.
He does not intimate that marriage is doomed. In fact, he specifies cogent reasons for marriage, which are the long-standing reasons anyone gets married: to share a life together. I know I will be reading his book to find out more.
In the gaze of the other 
While it is true that whatever a couple decides to do with their marriage is nobody else’s business, it is also true that most couples do not agree that each may have an outside sexual partner. One of the reasons these days is that all of us have to be more careful in choosing sexual partners because of the dangerous diseases in the sexual marketplace that were not present a generation ago. Also, while sexual mores change as does society in general there is one constant throughout all generations and iterations of society. That is the emotional aspect of knowing that your partner is off getting off with someone other than YOU. Mr. Ryan states that it is just sex, it let’s the pressure off, who cares if the couple agrees to it, etc. But in truth it takes a very special couple to have that discussion let alone agree to allow it. From personal experience my current love and I agreed to that in the very beginning of our relationship. We even openly discussed engaging in sex together with other partners. It seemed so exciting to meet someone who was willing to explore sex in different ways, settings, and pairings than what we each had usually done in the past prior to meeting. But as our relationship grew and our bond became stronger and we fell into love, suddenly that wasn’t so appealing anymore. When it was discussed after that it was more often dismissed because neither of us could picture ourselves watching the other receive sexual pleasure from someone other than ourselves. It was “just sex” as Mr. Ryan states, except that it wasn’t. Oh intellectually we knew that it was just sex, but our emotions and feelings told us otherwise. Now if the idea was discussed it was in emotional terms; a complete change from the beginning.
However, when a situation involving a couple changes dramatically or simply morphs into something that finds the pair diverging many will seek solace outside the union. It CAN let the pressure off but it can add more to it. Years ago my partner was a mistress and states that she is sure that being so saved her lover’s marriage; probably, she said, because it let some pressure off his marriage.
I don’t think it is wrong to enjoy a lover or even a love outside of marriage. Sometimes necessity dictates it, like when one becomes incapacitated for example. And sometimes it is just necessary, for very personal and private reasons.
I too was struck by the “just sex” comment, MPM, which I thought was rather oblique. Yes, there is just sex and then there is making love. It takes certain types of people to be able to be okay with a little on the side, but it happens more than you think, I would imagine. It’s all about the agreements.