I was told that because shutter speeds were slower in the early days of photography that photographers required their subjects to stand still for long periods of time to expose the image to the film negative, hence so many photos from the 1800's to 1900's without smiles in them! Luckily for us no one can tell.
See also the excellent animation at offsite link.
The only source of stimulation is the glans rubbing against the wall of the vagina.
I will try to do better next time Nik Silver Efex Pro 3, one of the standout components of Nik Collection 4, is a black-and-white conversion tool that goes far further than the grayscale or black-and-white tools built into all-in-one photo apps.