This is one time Jack would not be put off by Ennis. No matter what Ennis said, Jack was going to get his way. From the very beginning of this scene, Jack's face is set firmly. He is NOT going to let Ennis get out of this relationship at this point. Maybe Jack didn't know exactly where it was going, but he was not going to let Ennis say "no", in my opinion.
When Ennis said this was a one-shsot thing, Jack didn't flinch or back down one bit. He answered,"It's nobody's business but ours". He had spoken. Now he just had to wait until that night to see what Ennis would do. Had Ennis not come to the tent the second night, I don't think Jack would have given up. He was too set on Ennis to back off now.