Gay man here and I discussed this at length with my surgeons.
For full disclosure, I'm a bottom, so when I say I questioned the surgeon at length, I mean it. I even talked about options to the traditional ileo-anal surgery, talked about the option of keeping my rectum and my ileostomy indefinitely. None of those are good decisions for various reasons.
Anyway, anal sex after J-pouch is a definite no. Sorry.