AITA for grounding my daughter and canceling her senior trip after I found out she was cheating on her boyfriend? 

I have two daughters, Lizzie (17 F) and McKenzie (14 F). Their dad and I divorced a few years ago after I discovered he was having an affair. I have the kids most of the time, and their dad has them every weekend and during the summers.

Lizzie has been dating Jacob (18 M) for over a year now. Jacob is constantly at our house. He’s a sweet, good young man, and I believe he’ll be valedictorian of their class. However, a few weeks ago, I overheard Lizzie on the phone with a guy, clearly flirting. At first, I thought it was Jacob, but then I heard her say, “Brandon.” I realized she was talking to someone else. Then a week later, she mentioned to me that she was heading out to hang with a “friend,” and when I looked out the window, I saw her get into a car and greet a guy with a kiss. It wasn’t Jacob.

Even after that, Jacob continued to come over, hanging out with Lizzie. He and Lizzie still acted like a couple—holding hands, laughing, and spending time together—just like they always had. I felt disgusted knowing my daughter was being a two-timer.

After Jacob left that day, I confronted my daughter. I asked her point-blank, “Are you cheating on your boyfriend with another guy?” She said it was none of my business and that her personal life was hers only. I told her she was wrong and that I raised her better than to treat people like this. She told me she was bored with Jacob and that Brandon was more her type now. I told her that if she wasn’t happy, she should just break up with Jacob. She said she didn’t know if she wanted to be with Brandon or if she was just having fun flirting and teasing. I told her cheating was unacceptable and wrong, and as a consequence, I grounded her. I also told her she wasn’t allowed to go on her senior trip with her friends. She obviously did not take that too well and has been at her dad’s place for the last couple of days. 

My ex husband called me, saying I was being unreasonable not letting her go on the trip and that her and Jacob was just a “high school thing” He then told me I needed to put my “bitterness aside” and “stop punishing his daughter.” I told him I was teaching our daughter right from wrong, and that actions have consequences.