Thank you, both of you, for the advice.
I realize I need to communicate things more clearly myself - I hadn't exactly intended on having sex with M when I was up there, but it ended up happening anyway. I didn't think it would be such an issue, but then again, not thinking is what got me into this mess.
With M, he and I are putting our relationship "on hold", so to speak, until B and I sort out our communication issues. If I have to be more explicit with him, I'll do it, so he understands exactly what's going on - I know I don't want to lose either of them, as I love both B and M very, very much, so I'm willing to do whatever it takes to make things work on both ends of my little relationship V.