Hmm, why would an unknown creature from Earth let alone an alien one want to invade a female's anus and/or vagina?, that is why we have the lab. However, on various worlds and in still unexplored parts of Earth, the totally unexpected, happens.
Those who are unwary and some who actively invite these beast's attention find themselves being thoroughly penetrated vaginally and/or anally by unlikely lovers. Many bizarre experiences are possible and they happen here. So see those situations that have been observed "in the wild" where some lovely young thing finds herself in dire straits or desired situations.