I am from Portugal.
Swing is a good thing for many people i would say. Even if you think that swing is immoral, you must see it as a lesser immorality for a simple reason:
Its far away much better to have sex with other person with your wife/husband participating on that, than doing it on the "back" (i mean: "cheating").
My girlfriend is a woman that just wants me, but if some day she gets the desire of having sex with other man, i would prefer (even if i get hurted) that she told me that, than she getting sec with that man on my back.
Also, guys (and these is for guys) dont think that you wifeds ddont have sexual fantasys with other men...they have. At leats an high % of women (like men) secretaly wants to have sex with other men other than their hubbies.