It's offensive. First, like someone else said, it implies that natural birth is about proving something. I personally have no need to make childbirth about a challenge; I'd be perfectly happy to have it be smooth and easy from beginning to end. Wouldn't make it any less edifying for me. Doesn't make me any less of a woman.
Second, women get epidurals because they're necessary to allow them to relax, or because they're not educated and don't understand the risks, or because they don't have support for a natural birth and are psychologically undermined every step of the way (something that is very hard to deal with in the midst of labor,) or because the pain truly is torturous. Even if the goal is to prove something, none of the above is about being a "wimp". And until you've been through excruciating pain that made you feel like you were being drawn-and-quartered, you are not in a position to judge. If you have, you'll get it: people who think the only women who get epidurals are princesses who can't bear to break a nail in labor, just weren't pushed far enough to their breaking point. Trust me, there is one.
And yeah, Grantly Dick-Read. Well, fear and stress *are* complicating factors in birth for most women in our society. But that doesn't mean that when fear and stress are absent childbirth will be painless, because there are so many other complicating factors, and sometimes they can't all be avoided.