I’m a straight female in my mid 20s. I’ve been dating a wonderful guy for two years—but I recently found something that has put me on edge. Before we met, he was in a relationship with a terrible, alcoholic, and mentally unstable woman. They got pregnant early in the relationship and stayed together for about five years. We met a year after they broke up. I felt like I’d come to terms with the ugliness of his past, with his trying to stay in a bad relationship for the sake of his child and the rest of it. But recently, thanks to the vastness of the internet, I came across a suggestive photo of my boyfriend with his ex’s sister. I asked him about it, and he admitted to sleeping with her while he was with his ex. He says it was during a particularly bad period, he was...