If done right it's not really stretching but using enough lube and learning to relax.

And yeah what Aqua said. No need for an enema anyway if you have a good Bm before. Besides what most people don't realise is enemas and anal sex can be a bad thing. The enema washes away the natural mucus of the rectum which helps protect it from little cuts, etc. You almost always have some tissue tearing during anal (same for vaginal) you just don't notice it because it's so minute. But with an enema losing that protective mucus lets the bad germs into the little nicks.

(I read all that in a book I are smart)