It's quite common for men to have this reaction (it's called sperm competition) but less usual for women, although as swingers learned a long time ago, it's far more common than most people realize. While generic therapists may look for convoluted "explanations" that no doubt have roots in all kinds of childhood trauma and feelings of unresolved complexes, reality is much simpler. When we know our partner has been "elsewhere" we feel an overwhelming urge to "reclaim" them via sexual activity. It's perfectly normal and can be a very useful device in spicing up what otherwise could be a rather predictable and mundane sex life. At some point you may find yourself getting turned on by the idea of your friend licking you while your husband fucks her - this is a pretty classic reaction and can result (if the people are all ready for it) in some amazing hot sex.