The anus, whether male or female, is designed as a one way valve; it lets stuff out but resists letting anything in. The vagina, designed to admit penis is very elastic and can handle about anything. The muscles at the opening of the vagina are also designed to stretch to allow birth. Tears in the rectum and anus are quite common as people begin taking this action. These tears, of course, admit bacteria that cause infection and the stuff travelling through rectum is about as full of bacteria as you can imagine. The anal sphincters are not and can easily be stretched, even torn, to the point that they do not do their job.
