I agree with you that this is a load of BS, but only because IMO it doesn't matter in the slightest. His technical point is that an egg which is fertilized earlier or later than the optimal time is much less likely to survive. Any IVF clinic knows this well -- there's a very narrow window in which an egg can be fertilized and become a viable embryo, but for a time before or after that the egg will still fertilize but won't get past one or two divisions before dying. Obviously the clinics choose to ensure that fertilization happens at the optimal time whenever possible; what the guy in this article is pointing out is that the rhythm method involves choosing NOT to fertilize at the right time, even though this doesn't consistently equate to not fertilizing at all. Eggs have to be perfectly ripe to fertilize AND continue developing, but they don't have to be perfectly ripe to just fertilize and form a zygote.