When I read your OP @GEW my mind went to immediately thinking how I would feel if my DH told me he was going to do this, and it actually made me feel sick on your behalf!
I don't agree that there can be a reasonable reason for your partner to want to do this. If you haven't got children, then you should be the one who receives his foremost care and attention, and tho one whose feelings matter the most to him, just like your partner's feelings should be the most important thing to you.
I can't advise you on what your next actions should be, you and your partner are the only ones who know what the rest of your relationship together is like. So, all I can, or am willing, to say, is please try to keep calm, when thinking this through for yourself, and then try to judge whether you can trust your own instincts.
If your partner won't change his mind about the lunch date, and doesn't offer for you to go as well, then you need to try and listen to what your own mind is telling you. If what you are thinking scares you, then try to weigh up the pros and cons in your mind, or even better, write your thought processes down on paper, and then consider how you think you will be thinking about today's thoughts and feelings, and/or any decisions you may make in the next few days, in a year's time from now.
So, I can only reiterate for you to please remember that it is both healthy and important for you to take as much time as you need, to be able to trust your own thoughts and decisions. Of course, you also need to remain aware that your partner may have a very different time-table to you, and that he may not have the same level of integrity as you do. I am wishing you so much luck and friendly love, that things turn out for you in the best way, which you so obviously deserve. 🩷💐