Aintfoolin', I think you've summed it up here pretty good. Always amazed at how these guys turned Annie's word's into action and conveyed the back an' forth momentum of the moment with such brevity. I think you're right, Jack probably didn't expect to get f**ked that first night, there's usually a lot of steps along the way. But cowboy's ain't known for subtlety, and as Annie said, "Ennis ran full throttle on all roads whether fence mending or money spending" - once the ice was broken, or in the case, the pants were open, there was no stopping along the way. He was gonna' "see this thing through" to conclusion, whut's the point in stoppin', huh?
Later, next morning, when he realized what he'd done (f**ked his best friend) without being invited, even though Jack didn't resist at all and seemed more than willing to comply. I feel Ennis not only felt the internal conflicts of his own homophobia, but also feared that his action would be more than a little insulting to Jack. Jack, always the salesman, did a great job of clearing that up with his brief words about supper.
