It will hurt if she's not prepared for it.

Why don't you try getting her a SMALL butt plug, and encourage her to play with it? That will gently stretch her anal muscles, and allow for better muscular control. It will also do nice things for her anal/rectal area, if she does it gently. It won't hurt if she's used to things as big as your penis (eventually!) being inside of her. Be patient.

I also agree with what others have said: have her read with you what it will and won't do to her rectum and anus. Also, never, never NEVER force the issue!