something was missing... yes, I'd agree with that, and what was missing was an ability to man up and truly be present in this relationship.
I agree with this.
And, as for why so sudden - maybe an old girlfriend popped up. Maybe he's bi and had a thing with his buddy he went out with on Sunday. Maybe he's just a Love Avoidant with a low sex drive. (It never did add up that he was satisfied with such infrequent sex).
You deserve more, G. A smart sexy guy who wants to be with you and isn't satisfied with half a relationship or sex a couple times a month.