I have had it happen to me twice, once with a good friend, and once with an acquaintence. It sucked pretty bad both times. I would never do that to a friend. There are literally millions of other people in the world.
I could just not understand, especially with the good friend, how she couldn’t see how this might hurt me. I still had feelings for the fellow, because he is the one that broke up with me. He didn’t leave me for her but she knew how I felt about him. She was very popular and could literally have any guy she wanted, but she just didn’t think and went out with him anyway. She ultimately broke it off with the guy, because there were bout 10 other guys waiting in line to date her. But by then, the damage had already been done.
The acquaintence, and my boyfriend, literally went off behind my back and hooked up. I was not even given the courtesy of being broken up with first. They just showed up one day, hand in hand. I was mortified and heartbroken at the same time. I just wish people, both male and female would give more than a second of thought about how it might feel to the rejected person, and just not go down that road and choose someone else. None of these people ended up together in the long run, but the memory of the pain is always there, way in the back of my brain.