I am assuming that you are talking about infidelity.
For me, trust is paramount in a relationship. If that trust is broken I, personally, am unable (or perhaps unwilling) to allow that person to
earn my trust again. Once it's gone I am incapable of
giving it back. Maybe that will be viewed by some as a lack of
strength. But, the betrayer did not show strength when he indulged in an act that broke my trust, and presumably my heart.
When speaking of marriage, I view marriage vows as a sort of contract...an agreement of commitment. I take that commitment very seriously. However, when the other party breaks their end of the agreement, I feel that it voids the contract and that contract cannot be renegotiated. It's done. It will be painful, but it's time to move on.
As far as considering the other person's feelings...too bad. I will treat my partner with the same respect that I am treated. If he disrespected me and our relationship (vows if married) enough to cheat, then his feelings, at this point, are of no concern of mine.
I was married for 12 years and was betrayed. I walked away. It has been 6 years...have I forgiven my ex? No, I don't think I ever will. However, we remain friends.
That's just my view of infidelity and trust. Whether it provides insight, I don't know.
~NerdGirl