The boundaries of casual relationships

Part 1: I have been in a casual relationship with a guy for about 2 years. We have hardly spoken about our preferences about dating other people simultaneously or maybe looking for something serious in the future with mutual comfort. In the beginning I used to date other guys. He never mentioned of dating other girls and always behaved like I was the only one he's been seeing. At first of course I didn't believe him. But later on I got confused (not saying I believed in his story). During that time I had a little attachment with him all of a sudden. I tried to fight it because I sincerely do not see a future with him as we have different interests in our future. I am a person who believes that relationships should be sacred. As in you can date anyone you want (while informing the other partners) as long as you're single. But when you are committing to someone, you should be monogomous. I'm not expecting a relationship from this guy. But all I want is the truth. Is it too hard to ask? My first question is - When you know I'm cool with it, why did you hide the fact that you were dating other girls? Part 2- Recently he disclosed that he finally was dating other girls because he had to. Probably because he got to know that I know. I didn't confront yet. Now that we are clear with that part, now comes the challenge that he feels jealous if I'm with another guy. A couple of his friends intended to hit on me. Before I could say or reciprocate, he behaved like I'm off limits because he is exclusively dating me! Talk about hypocrisy! Question 2 : When a guy can date multiple girls, what is his problem if the girl dates other guys? What's up with the jealousy?