Thanks for your question. My name is Ryan and I'd like to help you out.
While I'm sure your girlfriend has good intentions it sounds like this problem is a result of her insecurities rather than something you have done wrong. If you are feeling like you can't be yourself at times because she will be offended, that is a sign that something isn't right. In a situation like this she needs to trust your intentions, and as long as you are not purposely leading someone on, or joking in a way that would be misleading in a romantic way, there is nothing wrong with being yourself and joking around with anyone male or female. In the example you gave, it doesn't sound like there was anything inappropriate that was said.
I respect that you are doing your best to be respectful of her feelings, so much so that you are avoiding doing harmless things like joking around with people for fear of her reaction. In reality, as long as you both have trust in each other, then you should both be free to joke and interact with whoever you want so long as nothing inappropriate going on. As long as you're just being a friendly guy, there is no need to adjust your behavior just because there may be a female out there who may take something the wrong way. If that's the case, the assumption would be that you would handle that situation appropriately, and that would be the end of that.
Overall it comes down to her having trust in you and your intentions. She is taking it personally, but this is an issue that would normally be unrelated to a person's relationship. You are obviously not trying to disrespect her. As long as you are devoted to her and she believes that, then you shouldn't have to interact with people any differently than you normally would since your intentions are good and your heart is in the right place. I definitely wish you the best with all of this, and if there's anything else I can do to help please let me know.