Not to even mention all the voluntary clenching and pulsing they can do beforehand. You fuck a butt right, or they ride right, and you can easily get the bottom off, and when they do, their butthole starts rippling and flexing in a way that often pushes you over the edge too. Okay, seriously, this is the first time I've seen an animation actually depict the bottom's rectum squeezing during orgasm, giving the top's dick that uniquely mindblowing massage that always makes me gratefully wonder why buttholes are such mutually effective sexual orifi despite being the 'wrong' hole.
