That's probably because those affected are becoming used to the feel and touch and the presence of another actual person there. Once those things get "boring" both parties will have better -endurance- and let the other, more sexual things get them to climax other than the new feeling of another body, another set of skin, another set of organs, and such.
At least, that's as far as far as I know.
We do what we do in life because the world tells us it's right. But what's there to tell the world whether or not what -it's- doing is right?