"I sent out a bunch of introduction e-mails and always end with "please respond, even if to say no-thanks".... nothing, crickets, nada"
You seem like a nice guy, but you also appear to be unaware of how many messages couples and women receive, and of how many are from single men who feel entitled to attention. By asking for a response (and by asking, I mean demanding, except politely), you are automatically putting yourself into the same box with men who feel that being on a swinger site entitles them to pussy. That's not a good look and will make this whole enterprise harder for you.
Unless and until you grasp that you probably won't receive responses no matter what you do, because the number of men looking for couples and women far exceeds the number of couples and women looking for men, you're going to continue to be unhappy with your reception on SLS.