I am not a vegetarian, but the usual rebuttal to this is that the standard Western diet bears little more resemblance to the ancestral version than the vegetarian alternative: the nutritional profile of largely sedentary farm animals, lots of grains, and cultivated fruits and vegetables doesn't look much like that of (say) wild tubers, berries, fish, and occasionally an antelope that you pursued into exhaustion and clubbed with a rock. This as far as I can tell is true (albeit complicated by the fact that we have little good data on what our ancestral diets actually looked like, evocative pictures aside, and also by the fact that the best modern analogues we have are fantastically varied), but it isn't a positive argument in favor of vegetarianism.

The analogy to sexual behavior is left as an exercise.