The problem is, no one should force morality on anyone, if I were to answer this question I would be forced to condemn what you and your spouse and those others did, but If I did that, i would be as bad as the Taliban, so I wont do that, I will simply say this much;
I would be sad that a relationship I swore before my GOD to honor and cherish til death do us part, would tarnish so quickly and be distracted by ideals of the flesh after a mere 12 years, that if in my eye I would have to look to another woman to excite me that was not my wife, I would be ashamed, that my wife would have to look to the flesh of another man to excite her would hurt me, in a marriage not just the phyiscal laws of fidelity must be honored, but take captive even the thoughts, for thougts are more dangerous than a mere physical act of sex, if you can remember it, since 12 years ago seems to be sooo long ago for you, if you can remember who you were and what you were thinking when you looked in her eyes and said, I do, you will quickly remember it was a thought that bound you together, the thought of a love so special that only you and she would share it, now here you are, betraying that thought with the thought of two tired musty old things, the bodies of your friends,, that will wilt and wither in the dust storm of time, but true love , shall never dim... for it lives in our hearts and minds, not in our penis and behinds :)
take care and thanks for the question, I hope we both got something out of it....