Well, I do think that the morning after, jack was probably more concerned with whether Ennis will repudiate everything that had happened the night before -- and shun him (Jack) altogether.
I knew someone in real-life who was actually "seduced" by an acquaintance with Ennis-like problems, and the acquaimtance later blamed my friend for turning him gay!
I don't know if it was so much guilt from Jack, as it was more a feeling of concern with Ennis reaction to him the next morning. Jack had guided Ennis through this, that's why Jack "waited" for Ennis to come to him on that hill.
By "topping from the bottom", Jack enabled Ennis to explore this part of him, and by letting him know, "Its nobody business but ours" , "You're not queer, OK fine", "This is a one shot thing, you got it." It was all safety nets.
Jack had to ensure that trust Ennis apparently needed in order for him to go to Jack that second night. It was about trust. IMO.
Wow, this forum is constantly coming up with new treasures to understand the movie.

Thank you everyone for yet another wonderful thread.
Ennis also blamed Jack, later, in the final meeting...it's because of you that I'm like this.
Yup. It's about trust. And to certain degree, a matter of moral responsibility. It was Ennis' first time. That's always special. Jack cannot, must not, and did not take it lightly. He respected Ennis to let him take the lead, without pressure, manipulation or mind games. He simply opened up himself, to be vulnerable to Ennis' rejection ridicule or even physical violence. Once he let Ennis knew how he felt, by grabbing his hand to show his arousal, he left the rest up to Ennis. As you said, manhattangirl, that was the only way Ennis would even consider SNIT, and with considerable deliberation by the fire, before opening up himself to Jack.
Jack had a responsibility to be genuine and gentle with Ennis at this point in their relationship. His, at least according to his own claim, was the position of experience and to certain extent, power. Ennis was the vulnerable child that he could have hurt easily.
Trust is never easy in any relationship. Once burned...