Good work on raising the stepfather seed of doubt--as you say, they do notice what you say, and gnaw away at it in their minds. He does need to think about what he'd give up IF he went ahead with a D.
As for accusing you of having an A--that's one of the standard WAS lines. It's meant to make them feel less guilty, and once they come out of the fog it's never heard again, because it truly is just part of making everything YOUR fault.
I don't think you should have to reassure him you're not dating. Knowing that you won't do it until you're D gives you the high moral ground and sets the right example for your kids, but you don't want to make him believe that you're always going to be there, the back-up, pining for him. If he asks, I'd say something along the lines of, "As long as we're separated, I don't feel that my private life is any of your concern." And then, in a friendly and breezy manner, change the subject.
Congratulations on not giving him the reactions he was expecting--you are coming along very well!!
