Frankly, out of the two, your s/o's behaviour seems stranger to me.
Maybe he'll figure out of his own that you talking to someone else about matters poignant to you doesn't take anything away from your affection for him and apologise for being silly. If he doesn't disabuse him of his idiotic position.
There is no problem. You had an interesting and stimulating discussion with a man who mistook your interest in the subject for interest in him, and your fiancé is being silly that you might want to talk to other people other than him about things.
Frankly, out of the two, your s/o's behaviour seems stranger to me.
Maybe he'll figure out of his own that you talking to someone else about matters poignant to you doesn't take anything away from your affection for him and apologise for being silly. If he doesn't disabuse him of his idiotic position.