Originally posted by AndrewBlack
Actually it is not uncommon for the external sphincter to become weakened by regular intercourse. You can only really appreciate this when someone is anaesthetized though.
I think a better term for it would be relaxed, not weakened. Weakened signifies that something will not assume its previous state, which, by my experience, is untrue. Go a month or so without having anal sex with a woman and her anus will tighten up again and become harder to penetrate.
I stand by my orignal replies.