The worst of positions is the woman on top of the man and he having sex with her on his back. This is in contrary to the natural position that Allah fixed for a man and a woman even as He fashioned them male and female. And this is damaging in that it becomes difficult for all the semen to come out, perchance some of it remaining in the member, to become spoiled and decay, causing harm. Also perchance some fluids from the vagina may run onto the penis. And also indeed the womb is not well seated from being turned over the semen, and to meet it there, and to unite with it, for the creation of the child.