Once upon a time I would have been unable to. Their cheating would have invalidated something which I needed. But having my needs dependent on another is a problem of my own making. I am not past that entirely, but I am beginning to understand it.
Ultimately I hope to get to a point where my needs are all fulfilled by being right with the world. In such a state, their cheating has no need to hurt me. I may still decide that they either don't want to or can not bring anything of positive value to my life, but how I evaluate them should be based entirely on wanting what is best for them.
From this perspective, their cheating can mean many things. It might be that they have an attachment to the other person, in which case they may need to be let go in order to explore it. It could be that they have needs which are being unfulfilled. The mistake in that case, isn't that they tried to fulfill them, but that we didn't have a strong enough relationship to figure it out together. Their transgression, than, is an opportunity to forge a stronger bond. It could also be a simple moment of weakness. This makes them very human and probably means they feel terrible about it. Punishment and anger does nothing for them. Forgiveness and love, however, do.
That doesn't mean trust isn't important, but we should want trust in others because it makes them a better person, not as a way to protect ourselves.