This is an interesting topic and one that I can tell you honestly, Jane and I have always struggled with.  As intelligent people, we know that keeping up the facade of a healthy marriage is critical to avoid the raising of antennae and suspicion, and this means having sex.  The fact that we "have to" makes it easier to take as it isn't something either of us want with our spouses...at least compared to the passion that we share between us.

But, as you point out, that doesn't necessarily make it less painful.  In fact, as our relationship progressed, we both started feeling the same way we would if we had been cheated on.

We went back and forth on whether to tell each other when it was happening or leave it unsaid and eventually found that it helped to know, even if it was painful to find out.  We had to know when and it did mean we occasionally skipped an opportunity ourselves because, at least for me, I didn't like to follow too recently (comparisons, etc.).  I just preferred a bit of time between.

I think you need to find your own balance, but I'd start by making it clear to each other that there is no right to be upset at your partner when they must be intimate with their spouse.  It's an important part of keeping up the appearances.  In an interesting spin on the old "she meant nothing to me" affair line, you must realize that the sex doesn't mean that your enamored or in any way threatens the relationship you are building with your lover.

Dick