You aren’t overreacting.
The guy that has been your boyfriend for several months has been on a dating app for a period of time, and did this behind your back. This is unacceptable. If your friend didn’t see his profile on a dating app, you still wouldn’t know that he is on a dating app.
Telling you that he is only looking for friends on a dating app is a complete lie. If he is seriously looking for friends, there are other more appropriate avenues to find friends online and offline other than on a dating app.
The only reason why he got angry with you after you told him to delete the dating app or you are done with him is because he wants to make it seem as though you are the one who is in the wrong.
In other words, he doesn’t want to take responsibility for his actions. By acting like it isn’t a big deal, he is hoping to make you question whether you are overreacting.
You aren’t.
There is no place for a boyfriend who is exclusive to you to be on a dating app. And no, the excuse that he is only looking for friends is lame.
The fact that he is only talking to girls on the dating app, and not guys, is further proof that all he cares about is connecting with girls on a romantic level on the dating app.
The fact that your friend discovered his profile on a dating app means that you cannot trust your boyfriend. You are better off ending your relationship with him and moving on.