Guys like Kellogg were pretty nuts.
Yes, although it was not uncommon to consider both sex and "gluttonous" meat eating as sinful at the time. The "sexual revolution" started to make sexual pleasure more acceptable and eventually seen as a good thing, but plentiful meat eating continues to be regarded as bad, perhaps more so now than it was then, due to the animal rights movement, USDA and AMA propaganda, etc.
In the West today and in traditional, non-agrarian cultures, sexual pleasure seems much less likely to be regarded as sinful than the West during the Middle Ages, Victorian and pre-sexual-revolution periods. Raw organs and raw seafood (such as raw shellfish) used to be regarded as particularly beneficial to the performance of human sexual organs and many older cultures still hold this view--even older agrarian cultures like Chinese culture.
Doc said my Dad was ridiculously fertile. He loved to eat lots of meat. His dad ate and sold meat regularly even through the Depression, and was very fertile too.
My Mom loved her vegetables. She wasn't the most fertile either. Interesting. Thank you. Something to think about.
In China, haven't people been extremely fertile on mostly veg? And India, many with no flesh?
I haven't seen study data, but from what I've seen of animal documentaries and anthropological studies of humans, while meat improves fertility for males, plant carbs reduce the time between pregnancies for females (and the practice in some cultures of fattening females with starchy tubers suggests that they may become more fertile with either of meat or very large quantities of certain starches). Some meat eaten by females seems to be very helpful to get females pregnant and create successful pregnancies (for example, when female chimps eat meat, both their procreative success rate and infant survival rate improve). With a plant-carb-based diet with minimal but sufficient meat, it may take more tries to get the female pregnant, but she can get pregnant again sooner, especially if not breastfeeding or exercising much--both of which are made possible by a sedentary lifestyle. There may be more pregnancy complications and birth defects, but even defective children can survive in a sedentary environment (especially as trades develop, such as shopkeepers, teachers, etc. that don't require much physical exertion or endurance) these are overwhelmed by sheer numbers of births. The ideal combination for frequent pregnancies seems to be large amounts of starchy plant foods with a significant amount of meats.
Probably most importantly, the sedentary agrarian lifestyle means that there is more total food available, so there is no upper limit on the population and it eventually dwarfs the neighboring hunter-gatherer populations. The agrarians can store masses of food, such as grain, which can be used to feed huge professional standing armies, which then subjugate the hunter-gatherers and hunter-pastoralists and either exterminate or enslave them. The traditional Chinese diet and social structure seems ideally suited to maximum procreation--a starch-heavy diet with sufficient meat (much less than Steppe pastoral society, but not pure vegan) in an extremely sedentary society made possible by fertile rice fields that were re-fertilized by silts from massive rivers plus animal dung, resulting in almost no upper limits on total population. India is a similar case.
Eventually the soil of the agrarian empires becomes depleted of nutrients, land use becomes maximized and water becomes relatively scarce. The agrarians then must conquer new hunter-gatherer and pastoral lands to get rich soil, timber and other resources. Thus, agrarian empires always either expanded their borders and/or acquired colonies with fertile soil, until the advent of petroleum-based fertilizers. Eventually the wars became as much or more over oil than soil.
This is an oversimplification of an extremely complex subject, of course, but that's necessary to keep the post brief. More scientific study needs to be done on this stuff, some of which is still speculative at this point, but it would be controversial and difficult to get funding for. No doubt Tyler will contradict much of what I've hashed together here in my attempt to briefly and quickly answer the question.
As someone suggested in another thread, many generations of modern-food eating, soil depletion and environmental destruction may eventually cripple the reproductive capacity of humans in much the way that destruction of habitat of a physiologically carnivorous animal sexually-depleted by a 99% bamboo diet has done to the giant panda, with catastrophic results for that species.