Essentially I want to know: Why do guys act like they want non monogamous relationships?
There was no "acting" here. He didn't act like he wanted a non-monogamous relationship. He outright showed you in actions that he didn't want one because he never committed to you.
Why did he have to act so mean if all I am is sex to him? Did he realize he gave a shit and freaked out or is he just done?
No All that conjecture is based on your own wishful thinking.
He was mistakenly embarassed by your behaviour because he thought that others knew you two were fk buddies so it would look bad on him if you're the one that is in control and he's the one that isn't calling all the shots. Like Smackie said, it's ego. Certainly not love.
but hey, we had really good sex.
Yes and sadly, you thought it was going to lead you to having a relationship with him. Sex does not equal love. Lust does not equal anything but lust. Actions such as being exclusive with you and asking you to be exclusive with him and actually showing you in words that match actions that he is committed to you and you alone. Sex is just sex if all you're doing is having it with one another, talking to each other through mutual conversation and not much more. That's just being a non-threatening, non drama causing fk buddy to him.
Read the book: "He's just not that into you" so you understand what its actually like when someone is just there for the ride and someone who is there because he loves you.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion