It can be for a number of reasons. Maybe the men is happy to see his wife's desire satisfied, plus he gets some kicks by fantasizing about her woman having sex with another woman. Maybe the gf is not entire homosexual and would let them indulge on a threesome from time to time.
Cases and nuances can be so many, it's difficult to explain. I am all for "live and let live", so long all of this is fully consensual and non-coercive/non-abusive.
Sorry, I did not know what church you belong to so I assumed Roman Catholic. I don't know how your choice of (Protestant?) church impacts your world view and how it might related to that of a Roman Catholic.
From what I have seen, very few swingers are in open relationships. Open relationships, I agree, tend not to last, for the simple reason that the level of commitment from both parties is, by design, quite low.
Most people into this lifestyle are actually couples with several years of married relationship under their belt, kids and mortgages and the lot, who choose to bring something new and fresh (and maybe a bit risque, too!) into their sexual life. Again, there can be a million of reasons. So long none of these qualify as coercion/abuse or is meant to mentally or physically harm, I don't see why not.