Is this like a "new" joke or something? i've heard it three times in the past month or so, though this is the first time online.
P.S. It's pretty lame and largely untrue.
I tend to believe constant use of the pooper will weaken it and when you're older you might be wearing diapers...
If i remember my human biology (and i don't, therefore i'm mainly talking out of my ass), the anus is normally contracted, which keeps you from shitting yourself as soon as your internal anal sphincter allows shit to fill the rectum. Short reponse: your asshole is normally closed so an hour or so of anal isn't going to hurt, unless you're taking a baseball bat up the ass.
And yes, "talking out of my ass" was intentional.