Took me a few years to figure out, but looking at this scene in particular I myself am astonished at how casually I can take a cock up the ass these days. And that comes in handy when one of the other holes in inaccessible for some reason. I just googled a bunch of recent sex surveys and found out that on average less than a third of women practice Anal Sex. I assume it's got something to do with the fact that the first anal experiences are almost always the worst. You're young, inexperienced, you skimp on lube, you go to fast. And like they say: Once bitten, twice shy. So you gotta get over that initial frustration phase and then just keep practicing. I guess it's similar to deep-throating which seems almost impossible at first until, after many hours of gagging on cock, you stop pushing the cock down and start swallowing the shaft instead. In both cases the actual skill is simply a mind over matter problem.