Not designed for it
It's actually not safe, because one's anus is not designed for repeated in/out, in/out.
Just like in any big city, some streets are one-way and some streets are two-way. A vagina is created or evolved to be a two-way street (penis in, baby out). However, the gastrointestinal tract is designed to be one-way, with an opening on each end. Food goes in the mouth, and the brown stuff goes out the butt.