Lostforwords offers great advice. I started out thinking that you can't make progress on your relationship while OW is in the picture, so maintaining a sexual relationship is just enabling cake eating.
On the other hand, to your point, if it makes him more conflicted and less likely to leave you for OW is it OK?
I don't think you can "out sex" an affair partner, even if you're "better" in every sense of the word, it's harder for H to let himself go with you than with someone who is less well known. That's something of a marriage tax on sex that has to do with personal development more than the skills of the partners. That said, I don't think this is your objective.
I guess from my perspective, there are 2 key questions:
1) What's it doing for him? Is it truly making him feel closer and more connected to you? OR is it just giving him a thrill, allowing him to satisfy a physical need, but allowing him to remain emotionally uninvested?
2) What's it doing TO you? It can't feel good knowing you are sharing H with OW, and maintaining a sexual relationship has the potential to be demoralizing as you will feel you're giving him all he's asked for and it's still not working. You risk the roller coaster of getting your hopes and expectations up.
I guess there's no simple answer other than to weigh the questions above and see if the positives outweigh the negatives.
Accuray
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015
