My dad is furious that my mom slept with other people in an open marriage he wanted.

My parents got married when they were super young. My dad knocked up my mom, and their parents married them off. My grandfather was able to set up some business for my dad in a big city, and they moved here soon after my birth.<br><br>My mom grew up in a conservative southern town where she was taught to be a submissive wife. And even after moving to the big city, she didn't spend much time socializing. She had no friends and never went out. My dad was only there to provide for us. He was always away on business, and he wasn't there as a husband for my mom or as a father to me.<br><br>My dad made a lot of money, so we never lacked anything. Growing up, I became my mom's best friend. We would talk about everything. I pushed her to make friends and to find hobbies. After years of pushing, she started going to a nearby park and made her first friend, a gym trainer. Encouraged by her friend and me, she decided to join the gym.<br><br>She met a few more people there and started having some semblance of a social life, but she still continued to tell me everything. And that's how I knew that she started developing feelings for the gym trainer. She was torn because she felt like she was doing something wrong, but the trainer was the first man in her life to make her feel wanted and loved. They never did anything physical, but they would go out together for coffee and other things.<br><br>My dad grew up in a big city and had a very different upbringing than my mom. While my mom was raised in a conservative town and married off young, my dad grew up in an entirely different environment. While he was on business, he was rarely faithful. I have two sisters, and I'm not sure if he's their biological father. I'm the only one that looks like him.<br><br>When my mom told him that she had feelings for the trainer, he wasn't bothered. He didn't see why they couldn't have open marriages. Honestly, I was a bit surprised that he offered this. I knew that he was cheating, but you don't always expect someone to be open about it.<br><br>My parents talked for weeks, but in the end they decided to trial it. They had some rules, but they were mostly allowed to do as they pleased. My dad continued to be away often, but my mom started seeing the trainer.<br><br>She was happy, and I was honestly a bit unsurprised my dad continued to cheat. I don't think he ever loved my mom. I felt bad for her, but as long as she was happy, I was good.<br><br>Then, COVID hit. My dad had to return and stay at home. He saw how much happier my mom was and immediately started asking about her and the trainer. She told him the truth: they were having sex, she loved him, and she was happy.<br><br>My dad had a meltdown. He screamed at my mom. Told her that she was a whore and that she needed to be faithful. My mom snapped back. She told him that she was allowed to see other people and that she wasn't doing anything that he wasn't doing. Then, she told him that the trainer had been making her happy and treating her like a wife for the past year, something my dad had never done. She told him that she loved the trainer and that she wanted a divorce.<br><br>The trainer moved in with us a few weeks later. My dad was sleeping at a hotel. He kept vainly trying to get my mom to change her mind and threatening to cut us off financially. My mom started looking for a job and so did I.<br><br>My dad was absolutely livid when the trainer moved in. He called the cops to have him kicked out, but my mom said that the trainer could stay as long as he wanted. Then, she told my dad that she wanted a divorce.<br><br>He refused to leave, but my mom had the trainer kick him out. I think my dad was so angry that he couldn't believe my mom was being so stern. He continued to threaten my mom, but she stood firm. Eventually, he realized she wasn't going to change her mind. He told her that she was being a bad mother and that I deserved a better mom than her. I screamed at him and told him that I think she's a great mom and that he's the terrible parent. He called me a whore and left.<br><br>I took the whole summer off from college to help my mom. My dad continued to threaten her, but she continued to ignore him. After a month of living with us, the trainer proposed to my mom. She told him that she needed to divorce first.<br><br>My dad tried to fight the divorce, but he lost. My mom married the trainer last weekend. I was the maid of honor. My dad didn't show up.<br><br>My mom's really happy, and I'm happy that she's happy. But my dad is the one that wanted an open marriage. I have no idea why he was so pissed that my mom had other relationships.<br><br>EDIT: Thank you everyone for your kindest words. My mom had a great wedding, and they're on their honeymoon right now. I'm still in the city. I'll let her know about all the comments tomorrow.<br><br>EDIT 2: Thank you for the gold and silver.