This is quite correct. If you make a promise to someone (which I understand is what many couples do at the altar) then you ought to honour it. And if you don't honour it, you have to ask yourself, first, why you would make it in the first place, and second, why the person who made the same promise to you should be any more bound than you are.The husband and wife have made an agreement. Breaking that agreement is wrong.
Consider, if one partner wants to play away, should not the other partner have equal freedom's or is the marriage build not only on deception but on hypocrisy too?
Interestingly, I've been with my lover for nearly 3 decades. We started long before marriage was available in Canada, and frankly, I no longer care about such formalities. And we have never promised fidelity to one another forever. And yet, for some reason or other, I've never done anything outside of our relationship. I don't think my lover has, either, but then again, it never occurred to me to ask him.