Honestly I couldn't come up with anything too god... As that's one of those situations in which you don't know how you would react until it happens to you.
But yeah, probably I'd feel pretty sad. Because if my wife/husband was cheating on my with someone else, I would ask myself what can that other person give him that I can't. So I could have myself to blame for it too (even if it was him/her the one who cheated).
It also depends how long he/she has been cheating on me. If it started recently, well, we could talk about it. If it has been going for a long time, then I would be mad about it. But yeah, picking a good moment to talk about it, and what to say would be really difficult. If I was really angry about it, I'd probably do it straight away, as soon as I can. Then if I wasn't so angry, and I though that we could actually work on it, I would probably start asking questions about it (pretending that I don't know), to get the other person to confess. And if she doesn't, well, there will get the point when she won't want to answer anymore and ask "what's up with all the questions?" Then I would tell her that I found out. And then there are endless possibilities depending on how the other person reacts, etc, and I don't know if I would forgive her (I probably wouldn't), but as I said, I don't know how I would react.
I hope this was the kind of answer you were looking for :)