Because semen swims upstream. The bladder is not located above the vagina (internally). If the ejaculation takes place in the vagina, the semen will swim up towards the cervix.
The best answer for this question would be:
biological and social
Medical anthropologist need to understand the source of their biological heritage and ofcourse the system of their bodies in order to understand their suffering. Social factor is also important, because it can tells them the source of their suffering.
