So pregnancy from anal is possible. Possible doesn't mean likely, but you carefully describe the mechanisms by which pregnancy after anal sex is possible. Two of them can happen to women who have chronic vaginal or pelvic infections. The third one, with the marathon swimmer sperm expelled from the anus, crossing the perineum, entering the vagina, reaching the uterus can be made even more unlikely by using a condom, woman supine, woman washing afterwards. Even oral contraceptives are not 100 effective. Nor is vasectomy, unless verified by tests a few months later.