Honestly, it seems as if the situation that you are in is not the best of all possible worlds here and could actually cause quite a bit of chaos. If you are okay with your husband putting these limitations on you about dating men, then you realize you will be walking a tightrope if you are allowed to flirt with men, but not go any further. Feelings may develop (looks like they already have) and you will be in the difficult position of being able and allowed to flirt, but not being allowed to go any further and date. If you are okay walking this tightrope with him, then that is your choice.
If you are really expecting/hoping that someday he will come around to the idea of you dating other men AND do not feel that this arrangement is fair or that you don't want to agree to it forever, then you are going to have to really address it between the two of you at some point. I'll give you a hint. He is going to put it off forever. If this is something you want, you will have to state what you need and want out of the relationship and that you would like it to be more fair. If he is allowed to date opposite sex partners, you think you should be able to as well AND you would like him to do the emotional work to start uncovering why he is so uncomfortable about other guys and what you and he can do to make him more comfortable.
In your current situation then, your husband is not agreeing to open up to other men, so you have to abide by your agreement and keep it to flirting, friendship, or nothing at all. That is your choice. If you think that this is something that needs to be addressed between the two of you, then you might want to address it sooner rather than later. He will never just change his mind out of nowhere. The idea of another man makes him uncomfortable and he has no reason to change that or explore it if he knows you will listen to him and not leave him.
I am sorry that he is so unwilling to explore the possibility of letting you date other men. I can't imagine I'd be comfortable in a situation like that and honestly probably would never agree to one. If he can do it, I can do it and if I can't do it, neither can he. I personally don't think it is very fair for one partner to have different limits imposed on them by another that gets to enjoy more freedoms.
So in summary I guess your options are to play along with your current arrangement, realizing that realistically he isn't going to change, or your can tell him that you need to revisit the agreement and that it isn't working for you anymore and start that process of trying to renegotiate what the terms and boundaries are.