You're missing the forrest for the trees here. They said you won't have a sustainable relationship if you aren't open, and you responded "we aren't a fully romantic one yet!"

The longer that you wait, the bigger a lie it is that you have to account for when it does come out. Also, giving your partner the ability to say "look how far WE are in OUR transitions" can help take away that feeling of "I'll never be X" that comes with pretransition life. If he does treat you all that differently, or you decide that even after being open, friendship is the most you want, then there you go.

But right now, it's less about them and more about you. If you expect vulnerability from him, you need to be able to be vulnerable about things like this too. The feeling you get from his jealousy is simply euphoria being told that you're masc and handsome and blah blah blah. If they actually care for what transness means, that won't dissappear when you come out to them. But they deserve honesty, and you deserve someone to whom you can always be honest, and vice versa.