With she on her fours he approaches from behind and entry, having total access to her anus. She can either stimulate herself in being penetrated in her anus or enjoy the ride in her vagina. And yet, his penis can be squeezed between her bum as she smoothes his testicles, a very exciting option for him.
In the heat of the moment, a couple becomes blind by lust and carry on obliviously to any excess, affirms the Kama Sutra.