Adding my perspective:
I want to throw out a caution here with regard to S, due to an experience I had where I really dropped the ball. I (being monogamous) was struggling with the fact that my wife (very monogamous) was showing signs of Alzheimer's (complicated story), she was struggling with the fact that I was rebelling against everything, and along comes a polyamorous dynamic where I fall in love with another woman. Well, essentially, a struggling marriage (my wife and me) combined with NRE (the other woman), and I had a hard time remembering the positives about my marriage. I was very gung-ho about the new romance I had just discovered.
And the "new woman" was a perfectly good person, there was no problem with putting her on a pedestal. The problem was bumping my wife off the pedestal -- something I did in my own mind, and something I deeply regret today (and there's nothing I can do to make up for it -- my wife passed away last year). I just can't warn strongly enough about how powerful NRE is. I know you and D have been together for at least a year, but I swear that NRE can easily last that long. D probably feels it for you, but it's probably been a long time since he last felt it for S.
In the meantime, S is the "chore relationship" -- the relationship with kids to tend to, jobs to do around the house, bills to pay, and on and on. Whereas D's time with you is probably free of all those doldrums and minutiae. You and D = pure romance ... whereas S and D = the depressing realities of domestic living. I don't mean to presume I can jump into your minds and know that's how it really is, but I'm just saying there's a reeeally good chance it might seem that way in D's eyes. Which could be the reason he's thinking, "I've got something with Araneidae that I never had with S."
And actually, that's technically true. What he has with you is different from what he ever had with S. That's because you and S are two different people. Two different people --> two different relationships. Comparing the two is like comparing apples to oranges. The day may come when D lives to regret bumping S off the pedestal. The truth is, you and S probably both deserve to be up there.
I know the situation I had is different from the one you have. First of all because yours was *initiated by* "the wife," whereas mine was *introduced to* "the wife." And there are many other differences, I don't doubt. I'm just saying be really careful about the NRE factor, because it can make the whole world look turned upside down.
Other than that, I'm in agreement with all that's been posted on this thread, and I really like the advice GalaGirl's had to offer. Try to work on the problems a little at a time, and make a new resolution to try to be friends with S -- on S's team, not on the opposing team. You need to know that S is on your team, and S needs to know you're on her team as well.
If I am out of line with anything I've said here, I apologize. I guess I just hope others will be able to avoid the regrets that I ended up (justly) experiencing.
Love means never having to say, "Put down that meat cleaver!"