"Nicely"??? Yes, I don't think you VILLIFY, and you let them know that you love them, with REAL love, not the fake, romantic stuff that will soon wear away but with an EVERLASTING love. Be firm, but loving.
But "nicely"???
There's nothing "nice" about a spouse's decision to have an affair, and I think they lose respect for us when we don't stand up to them firmly and fight for our marriages and our families.
