I’m new to polyamory and I’m not quite comfortable with my partner cuddling, flirting, etc with other people in front of me just yet. I’ve talked to him about it, but he doesn’t see what the big deal is and calls me “crazy”. Am I being irrational?

You are absolutely not being irrational. Your use of the language “not quite comfortable just yet” indicates that you’re doing the work of self-awareness and self-compassion that healthy polyamory takes. You know what work you need to do, and you are doing that work, but you’re also being honest with yourself about where you currently are in the process. Then, you communicated those needs clearly to your partner! All of that is totally awesome, and it’s something your partner should be proud of and encouraging. I am sorry to hear that he has made a far worse choice, and is invalidating your feelings in a really callous way. 

Anyone who brushes off your feelings and calls you “crazy” is not practicing good communication or healthy consent, and I’m wary for you as a new poly person who’s being treated like this. My recommendation is for you to firmly and clearly remind your partner that you are working on all this, but you deserve patience and understanding from him, and that he needs to make accommodations to help you get your needs met just as you are doing for him. If he refuses and puts your feelings down again, that is a significant red flag that he may not be as good at this as he thinks he is.