Based on the manga by Koume Keito (小梅けいと). In this alter reality sex is not a reserved theme not even a discrete thing. Due to the existence of female and male pollinic humanoids, they travel just like pollen via the air and just when they find a receptive human they start their process of fertilization or at least thats what they intend to do. It's just normal to see them everywhere and everytime, there are only few ways to avoid them, so almost anybody is caught up.