My husband and I agreed to an open relationship since the start of our marriage. It's worked great for us! We have ground rules that we maintain (no getting emotionally attached, no actual dating of people, etc.). Well, I started seeing this guy from work. He claimed he was also in an open relationship with his then-pregnant fiancée. So we messed around quite a bit over the last few months. After about the 12th time (a few days ago)  he tells me he isn't actually in an open relationship; that he lied to me to get in my pants. I told him that he better tell his fiancée what we've been doing, or I will. Come to find out, he's been trying to get with just about every single girl in the office. This made me even more mad, since I know he doesn't use condoms (I forced him to with me, and he fought it like crazy). So he's been fucking around with who knows how many other women, and more than likely not using protection for a significant number of them. This puts all of these women, myself, his fiancée, and his at-the-time unborn child at risk of STIs. His fiancée went into labor and just had their baby this morning. She wasn't due until early October. But He still hasn't told her. Do I stick to my threat and get in contact with her myself? Or do I let them be and just hope he stops?