Thanks, JKelly, Autumnal and Mono. JKelly, you're right, it is entirely up to Stephen. While I had voiced my concerns, the one statement I had not expressed to him was, "I think you should end it." Your comment forced me to mentally prepare myself for an entirely fair refusal.
Autumnal, I think for the last few weeks, Stephen and I have just been conveniently ignoring the elephant in the room. When I did tell Stephen my direct opinion (as opposed to suggestions, observations, concerns), he said that he does think it's counterintuitive to be honest in our relationship, and then be so dishonest in a separate relationship. He's agreed to end it, and in the future, we both agree to see people who are more like-minded. Easier said than done, of course
I never thought I was relationship-savvy, believe me, but sometimes this feels like Relationship Preschool. Basically, we're very idealistic ... and we don't know what the heck we're doing when it comes to involving an entirely separate human being. I just keep reminding myself that like any meaningful practice, it's a process.
Thanks again, everyone - I'd just like to say that it's nice to have an outside perspective, without judgment. I haven't really sought advice about my relationship - it's something Stephen and I have always discussed just between us. It's great, don't get me wrong, but it probably isn't great for everything.