It's happened to me a couple of times in the past and to be honest I was that devastated that looking for someone else to have sex with as revenge never entered my mind. I suppose it depends on the relationship. In a casual relationship based more on sex and fun rather than love, it would be easier to just think "fuck it, let's see how you like it". In a deeper relationship, it's about the breaking of that trust that hurts. That's why I maintain that couples that swing have, in general, more reliable, stable and loving relationships because they are honest and can separate love from sex.