When I was a teenager I'd never even thought of this.......

.........until I saw a crude scribbled drawing on a shithouse wall of two men in this position, beside which had been written (with several mis-spellings) "Dubel suking. Beutifull int it?" and in someone else's writing underneath - "It's the best thing ever" It caught my imagination and haunted me until I eventually found a man who took me back to his place and we could do it for my first time, though not his of course. The recommendation on the shithouse wall turned out to be absolutely right, and I've never looked back since.