It won't stretch the anal sphincter any more than having bowel movements can. And nobody's anus is bigger because they have had bowel movements over time. The sphincter is made to stretch and then retain it's shape.
People who practice "extreme penetration" with things like two liter bottles and huge dildos etc may have anal prolapse, but normal anal sex, either with toy or penis or finger will not "stretch" and the anus permanently. You can learn to control the anus, so penetration is not only not painful, but pleasurable, but it doesn't "Stretch." The human body just isn't made this way.
Think about it. You have all different sizes of poops come out of you. Your anus stays or goes back to the same size after the bowel movement. That is what it is made for. You put large food into your mouth, does your mouth "stretch" and stay bigger if you like to bite big sandwiches? No. The anus and the vagina are actually as, if not more able to snap back to their original size and shape than the mouth, so she doesn't need to worry.
Take it slow, use a lot of lube, stop if she says to, try small things first (starter plugs or small finger) and many women and men do find they enjoy anal penetration a LOT.
Add vaginal penetration to a woman and WOW! Some of us don't know how we lived without it before we tried it. It is really an intense experience, and I feel even closer to My Man when we DP.