He may not really have reasons, Garlic. He could have been happy to go with your flow - until he wasn't. Until that switch flipped. He fancied a flat, you wanted marriage, he went along - now we realises he's married, ttc, and maybe he just doesn't like the responsibility, the pressures, the growing up.
It's OK for you to have wanted marriage, or a sofa, or children. Without hanging back until those things occur to him.
I don't think he wants to work on the marriage, I don't think he's going to surprise you with anything positive.
What comes next? Will he do the decent thing and move out - or dig in his heels and try to wait you out?
Ultimately, do you want the flat? Can he buy you out? Or must it be sold?
I hate to think of you forced to share living space with him while you try to mourn and get your life back together.