I have run into plenty of folks who broke up after the first affair. To a person, they all said that “it wasn't the affair, it was the lying”.
Trust is the basis of marriage. Carrying on sexual relationships with others, with or without a spouses knowledge, undermines that trust both consciously, and perhaps more importantly, unconsciously.You start to hedge yourself and your investment in the relationship.
“Trust is the basis of marriage.”
I believe that to be true of any relationship, from coworkers to friendships to dating to marriage. Once someone has betrayed that trust with me, I find it is nearly impossible for them to earn it back. I have not yet been married, but in the relationsips where I have entrusted them with my heart, their abuse of that trust has been devastating. Inevitably they ask for another chance and I find that I can’t stomach that risk again.
I've seen a whole lot more of the fall out than the improving from an affair.