My wife has enjoyed being with both friends and strangers for several years. Kate loves the excitement of spreading her legs for a guy she has just met. I love watching guys enjoy a one-night-stand with my wife. There is something very exciting about watching a total stranger undress my wife, taste her, and fuck her.
On the other hand, I really find it erotic to be around guys we know in our everyday life who have fucked, or who are fucking, my wife. Them fucking my wife is rarely ever part of the conversation but it provides a very erotic undertone to the friendship. Kate finds it hot to let the husbands of women we know fuck her.
Maybe we have just been lucky, but with every married friend who has fucked Kate, our relationship with that guy has become closer. We have never had any problems with it.