I was chatting with this guy, nothing happenend so why cant we still be mates?

Most people would be upset when they find out that their significant other was chatting up some other person... yes it is about trust but at the same time it is also about respect and boundaries... you made it clear that you guys are just friends and I don't know what the context of your text was but maybe your bf felt threatened by this other guy... how would you feel if you caught your bf texting another girl? Basically just put yourself in your bf's shoes.