GAG, wasn't it H that actually mentioned the M word about breaking up because GF wanted to get married? Then he gave you the other information, in response to your questions.
I'm wondering if XH has responded to your questions and then later on reflected on your conversation. He may be thinking that he gave you too much information and an insight into his real feelings. He may not even be sure of his feelings and some of your questions about keeping secrets may have made him feel uncomfortable. If he sets this boundary he can still enjoy your company without concern that he'll reveal too much of himself or have some uncomfortable internal conversations. Hope this makes sense......