What's "natural" (think cave people) is for a girl to get pregnant at 14 or so and keep popping out babies until she no longer can (and that usually wasn't because she got old enough to become infertile). Women weren't built to have fertile years x 12 periods per lifetime. Women were built to constantly have babies. In the strictest sense, even saying "no" is unnatural.

Love, Columbine (wearing clothes, of all things, in 80-degree weather, but only until she indulges in some of that creepy newfangled bathing)