And I think you should redirect the thoughts you have about their marriage and what she's like, or knows or thinks, and instead concentrate those thoughts on yourself.
I get where this is coming from; the problem is, that's what I've been doing up till now - ignoring their marriage and (selfishly?) doing what I want with him. If I ignore his lies and neglect of his wife I'm content with our relationship as it is. The problem is the vague guilt... kinda like dealing drugs? The transaction itself is consensual, but the predictable result is eventual harm to others. But maybe you're right it is not my fault / not my business to care?
That's not really what I meant, in either case.
About focusing on yourself: that doesn't mean "focusing on what feels good to you at the expense of others" - not even close.
What I meant was, for example: focusing on building up your poor boundaries. You started this affair at work, and you identified that one of the things that increased your intimacy with a married man was that you shared your medical history and some connected personal information. This opened the door to more intimacy. Clearly, as someone who will be in the work force a long time, this is something you'll need to address in yourself, right? You'll have to have the conversation about potential medical issues over and over again. How will you do this without creating a sense of inappropriate intimacy next time?
Or take this:
The transaction itself is consensual, but the predictable result is eventual harm to others.
So your part in this is that you have the ability to consent to a transaction that harms others. Or you have the ability to consent to transactions in which a third party is not harmed (either all parties consent to a multi-party relationship, or you are in a two-party consensual relationship). Which would you like to spend your future investing in? Probably the non-harmful relationships. So, looking at yourself, focusing on the part of yourself that makes those decisions - how can you improve those decisions? Is there any part of you that feels good about being in the harmful (to someone) relationships? If not, then how can you improve your personal boundaries to feel obligated to say no to relationships offered to you that harm another?
That's what I mean by focusing on yourself. Focusing on improving your boundaries. Focusing on developing a criteria for relationships that allow you to be who you are, but not at the expense of someone else's safety or well being.
But maybe you're right it is not my fault / not my business to care?
Of course part of it is your fault. You say you suspect something you're doing is harmful and you've done it anyway. The "fault" part is about you, not about the larger context of their personal relationship with each other or the choices the BS and WS make in their marriage. And what I said is it's not your business to fix their marriage. Your choices impacted their marriage, and you can cease that impact by stopping the affair, but IMO at this point your actions should not be primarily motivated by what happens in their marriage.