So your H and his AP might have had great sex because it was all new and fresh, but it wasn't competing with anything else in their life.
Exactly. Please try not to dwell on this. Their sex took place in a false bubble, undisturbed by kids and a million other "realities." NEVER compare cheap affair sex with committed lovemaking in a marital relationship borne from honesty, friendship, etc.---in other words, healthy reasons.
This is why most relationships that begin as affairs are doomed. Their genesis are lies, deceit, and escape. Rarely will such relationships last.
Wishing you strength--and perspective.