However it is surprisingly common and successful for many of those who go down the road ! I think there is actually some biological basis for having such desires having to do with diversifying the genes in a group. We are mostly monogamous but with powerful tugs to wander (both men and women).
Conclusion : Couples who go for swapping generally lack an emotional connect with their partner and have trust issues in the relationship. In the long run, these people get a feeling of chronic emptiness, which may get them addicted to alcohol or drugs. They may also fall victim to depression. As for couples who are parents, it can be disastrous for their children.