Same question so I'm camping here ..^^
Im going to take a wild guess and say its the exit of ones large intestine, the angle that you see.
If you look at the large intestine anatomy it doesn't have that. I fear that is just added for the sake of horny. Because it looks very closely to what you would see a cervix to look like, and the scene it self gives hentai catering to a breeding fetish (the way a/b/o does). I had to go back to the story description bc I thought I missed some A/B/O dynamic or something lol when I saw that clip. It has folds(transverse folds) but it doesn't really have any walls to breach. I could be wrong but I've taken an anatomy class, and double checked a large intestine diagram bc i thought i was trippin' and didn't find anything. From personal experience of someone who has done analysis a few times i can't say I've experience that either. The feeling of fullness and pressure on your stomach 100% is a thing but not the faux cervix lol.
Done anal*
Lol analysis is hilarious
Okhay .. Thanks!?? ლ(´ڡ`ლ)
No problem. As and ye shall recieveth ヾ(❀╹◡╹)ﾉ~
ALSO @ anal sex connoisseurs, what was that barrier bro broke through??