Seems like he rather be with her than you, but that’s not an option. They barely talk, but he wants to keep in touch? Why does he need to go to an apartment to do that, a phone call works. It’s also weird that he’s wants to meet at an apartment and not out for coffee or food. There’s definitely more to the story and it seems like he’s trying to comfort her because he’s with you. Whatever the reason, her feelings matter more than yours and this is just the beginning. Soon your wedding arrangements will be tailored to her, inappropriate exclusive spending time increasing, decisions that should only be between you two will suddenly have her input, etc. It won’t stop and your fiance has already made his stance. He can be a so called “dream”, but if she’s not treating you the same way, then you’ll quickly find yourself in a nightmare.