I don't believe in jealousy and never did. If something's meant to be, it will be, and if it's not meant to be, then it won't. If a relationship's meant to last, it will, but if it fails, then there was something wrong. Similarly with extra-relationship sex, except that this is extremely common. Very, very common, and only a small proportion are pre-agreed open relationships.

The most common reaction after an affair is the faithful party feels as if they've failed in some way. An affair doesn't mean that, because we're biologically inclined to be attracted to others, regardless of our core relationship. The least common reaction is divorce, unless the relationship was failing in the first place.