Suppose you have two married couples who are extremely close friends with each other. They all four agree to swap wives for the night due to the tight emotional bond the foursome have. Nobody has any regrets the morning after, and both marriages and all friendships remain intact.
The reason I pose this situation is wife swapping in the modern age can sometimes occur under these conditions and therefore the situation is relevant to whatever standards of the Law might govern it.
What I'm precisely asking is this: Is adultery defined as the breaking of trust between husband and wife by a third party acting upon either, or is adultery defined by the mere physical interaction between a spouse and a third party even when the other spouse gives permission?