I guess it depends on a lot of things: is if a 10 year relationship? A 10 month relationship? Was it a one time thing or an ongoing side relationship? Did the person confess because they regret or did you find out by yourself?
These are all factors that come together. With time, I think, all relationships reach a crisis of some sort and some people might get confused and make a mistake, but if that happened 1 or 2 years into the relationship to me it'd be unforgivable. Also if the person told me because they regret it I'd probably at least try to work it out (but I don't think I'd be able to) and so on.
Either way, I don't think it's worth the effort to try in most scenarios. There's something that broke between you that will never be there again