Speaking from what I have learned through various sex advice sites and televishion shows, there is no harm in anal sex like what you have mentioned, the muscles will "fall" back into place after they rexal or tighten as the case may be, on their own in a short ammount of time. They have to allow for various sizes because of the human waste that must pass through. The walls of the anus can tear with vigirious thrusting and rough treatment, this can lead to bleading and because of the location and bacteria be slow to heal.
I hope I helped you, if not please over look all sex advice from the virgin lol

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