An update: K has definitely felt guilty about the whole thing, and has apologized for causing me pain. I met with both K and co-worker to talk things out. Co-worker is in college, and not out to her parents, with whom she still lives. With the understanding that everything would need to be kept on the DL, I nervously gave them my blessing to see each other romantically, but requested I be kept in the loop. Everyone agreed that if this turned out to be too painful for me, then they could shut it down.

They spent the night with each other earlier this week, and while nothing happened, it was still agony to spend the night alone in our house. Our work schedules are pretty opposing as well, so it really came out to almost two days of us not seeing each other.

Yesterday, however, the co-worker called K to say she wasn't emotionally ready for a romantic relationship with her. She explained that she's recently out of a 4 year relationship, and realized she hasn't really unpacked that. So for now, she wants to be, well, a college kid: sleeping with people casually, but not forming real romantic relationships.

K is pretty heartbroken, and I've been consoling her, but it's difficult. We're both sort of angry that this girl introduced a big and painful topic into our lives, potentially jeopardizing our marriage, and then dropped it. K said some pretty hurtful things without meaning to, along the lines of, "She gets to go out and sleep around and have no pressure, but I have to come home to my wife." She immediately realized how crappy that was and apologized profusely, but I'm still simmering over it.

I kind of don't want to be her emotional support. I'm not even sure how to support her through what's basically a breakup with her not-even-a-girlfriend. I feel bad about that, but it hurts to see her still pining for someone other than me, and so I don't know how to help her, but I don't want to be cruel, either.

Our relationship is still healthy, and we've had a lot of very open, serious talks, but this is... I don't know, a lot. I'm totally drained. I'm losing productivity at work and making stupid mistakes because I'm distracted and exhausted by this.

I suppose I'm mostly venting at this point, but I appreciate everyone's responses. It's at least nice to be heard and legitimized to some extent.