Fake love

I just have a question ladies how can you drop someone after you tell them that you love them and want to start a family and grow with them and then sleep with another person in your bed. Like how do you not even have the respect to talk to me so I know what we are before you do all this but he fucked her while I was still under the impression that we were still dating. And even if we weren't how do you move on so fast from someone that you supposedly 'loved' like they were nothing. Like it just doesn't make any sense.