The common mode of sexual intercourse is not even natural. Our genitals are not for pleasure, they are for procreation,  and that occurs when two people are very much in love and wanting to reproduce. Nature takes it course when the couple are asleep laying naked and embraced. Procreation occurs by the vagina acting as a vacuum, drawing the flaccid penis inside to a climax and, eventually, ejaculation. 

The vagina then releases the penis, all the while not disturbing the peacefully sleeping couple. 

Forceful sexual intercourse is unnatural.