I cheated on my bf last year, does he still get the right to mistreat me?

Last june I cheated on my boyfriend. The worst mistake ever. My boyfriend didn't want to go to prom with me, so I went with another guy. It was just 2 kisses nothing more. ( They weren't open mouth either) I juked with the guy and that's it. We never texted before or after prom. ( Just like acouple but about prom.) It was just that one day mistake and my boyfriend found out about everything. That was last year and when he gets mad he calls me names such as a "busty". Says things like " you should be shitting bricks that I haven't found anyone else.." In reality he took me back we decided to work things out. So why this now? He also wants me to take a lie detector test, of wich I have to pay. Just to make sure nothing else happened. Some people say I shouldn't pay for it if he wants it he pays for it. its $450 FOR NOW HE SAYS I HAVE NO VOICE UNTIL I TAKE THE TEST. I've changed everythin bad, I dropped my friends, deleted fb and myspace, got his initial tattooed but nothing has changed.