So, only 'natural sex' is allowed in your house, like they do it in the wild: sneak up behind, hold 'em down. 
Is your approach banging down the door while growling, 'don't move or scream, I will be done quickly'
Is the number of children you have also the number of times you have known your mate in biblical way?
That is how it was 'naturally' for thousands of years
To each their own, just quit being so dang creepy about visualizing others doing it, m'kay?