Why do guys think its okay to hook up with girls right after they break up?

So me and my ex-boyfriend just recently broke up and i started talking to this new kid after like a week we just texted and talked… My ex found out and got really pissed and called me and told me how he couldnt believe i was over him already and it made him think that he didn’t mean anything to me… then he decided to have sex with this random girl a week after we broke up. My brother ended up telling me what happened and i called my ex and freaked out.. my ex acted like he hadn’t done anything wrong and that it didn’t matter because we already broke up. Now i have no respect for him anymore as my friend. Am i wrong?