My (F23) roommate (also F23) and I have lived together for a year and get along incredibly well. We both work at a large hospital in our city and have bonded over that and many other things. In the winter, she began to mention a guy at work who she had a crush on but he had a girlfriend. This guy (27M), we will call him Joe, works in the same department as her in the same role, and so does his girlfriend (F26). All I know about this girlfriend is they've been together for two years, live together, and have a dog together. Soon after my roommate first mentioned him in the winter, I noticed they were texting A LOT (specifically over DM), and I made a few comments here and there but she told me it was nothing. Towards the spring she noted having feelings for him, and I was very blunt in telling her he had a gf and there wasn't much to be done. In the beginning of the summer, my roommate and Joe go out with a few coworkers to a bar, and he walks her home at 2am. Note both were sober, but they sit on our front steps and talk for a while, and he confesses he's miserable with his GF, and essentially confirms he also has feelings for my roommate, acknowledges hers towards him, and leaves it at that. This continues alongside the VERY frequent DMs, flirting at work, and constantly mentioning him. She confesses to me about two weeks ago that while I was on vacation, they both happened to be at a popular hiking trail at the same time, one thing led to another and they kissed, then fully made out. She was giddy to tell me, and said verbatim that she doesn't feel guilty at all for him having a gf. Note she is well aware I got horrifically cheated on in my last relationship throughout this whole ordeal. I am very honest with her in telling her I do not approve, its a bad idea, and he's not a good person, despite saying he planned to breakup with his gf, and she continuously asks if I think less of her as a person. Tonight, she tells me Joe kissed her at work today in a supply closet and I kinda snapped, telling her its irresponsible, he STILL has a girlfriend, and that hes not given any good reason as to why he can't breakup with his girlfriend to be with her, and even then what prohibits him from cheating again. She told me Joe and hers situation isnt that black and white, but I truly cannot see how. My roommate hasn't said much to me since, but I know I am the only person in her life who is adamately against the relationship, and she told me our other roommate (F22) who is a co-worker at a different job of Joe's likes him and approves. So AITA for not supporting my roommate's relatoinship?