Affairs are a BETRAYAL -- some would say, the ULTIMATE betrayal, because it comes from someone who took a vow before God and all your friends and family to forsake all others for you.
Would a true friend betray you, and be unrepentant about it? Would you want to "preserve the friendship," and be "amicable" with such a person?
Civil, yes. Amicable -- sure, I can see amicable. But a FRIEND? Unh-uh . . . friends don't do that to you, I'm sorry.