I just grappled with this very thing, and I have to agree that every sitch is different, and every relationship has its own dynamics in play.
I think Julie has a point--it doesn't seem like he is cake- eating. He is struggling with feelings and hurts, and maybe needs/wants the sex for validation that she still loves him. I know in my own R, in the past, that was the only time I felt H loved me.
And only Klassic knows what is in her heart. How does it make you feel, especially afterward? If you are feeling used, or like you said, you feel bitter and resentful then probably it is not a wise choice.
As I said, I just had to make this decision, and didn't even think it was an option for us, as my H is the one who is leaving. But he approached me, and I felt his motives were genuine, and that it was something we both needed, at the time. And I did not feel angry or bitter or even regretful afterward.
My answer could change, and I will reassess, should it come up again (no pun intended) :P
Me 47 H 49 S18 S15 M 21 BD #1 11/09/15 ILYBNILY I believe we are Piecing 1/2/16 Suspect EA/PA? 2/28/16 BD #2 "He tried, but needs passion." 2/28/16 Confirm PA 3/11/16, he's leaving in June H leaves 5/7/16