Okay, just a few thoughts on the subject that may be helpful. Given that this is an internet forum, I cannot possibly take the time to write out a thoroughly cited research paper. So, if you are not content with my summary contents, just email me and I will direct to useful sources, in case you want to do in depth study yourself. I've spent quite a bit of time studying this issue. Here it goes:
First, it was the nearly universal opinion of the early Church fathers that sex was only for procreation. Any sexual activity undertaken in order to take advantage of infertility (such as sex during pregnancy and mensus) was frowned upon, sometimes with canonical penalties. The fathers were opposed to all birth control, including what is now called NFP. In fact, St. Augustine in a letter against the Manicheans explicitly condemns the rhythm method. Even in Roman Catholic circles, one does not begin to get the notion that using the infertile period is not sinful until the late 19th century. Humanae Vitae was revolutionary in explicitly endorsing the so-called "natural" methods of birth control. For the vast majority of Christian history, there were only two choices; have intercourse and accept children when they come or completely abstain. In the early Church, post-menopausal couples were even encouraged to be celibate.
What does this mean? It means that no Church today follows what the fathers taught; except the really strict traditionalists (both Orthodox and Roman) who are opposed to all attempts to avoid conception, even NFP. And, really what is the difference between NFP and non-abortifacient birth control (through condoms or whatever)? How is using a thermometer or scientifically checking one's bodily fluids "natural"? The fact is that one is still using human reason (cunning) and technology in order to take advantage of an accidental state that happens in nature. The intent is no different.
So, if one is to be consistent, one must either condemn all forms of birth control (including NFP) or one must permit any non-abortifacient form of birth control that does not have other moral problems (obviously masturbation, etc. is out of bounds). I can respect the rigorist position, even though I don't agree with it. My own view is that the fathers did not adequately distinguish between abortifacient methods and non-abortifacient methods; they were overly influenced by Platonism and Stoicism; and ancient contraception had its roots in paganism and sorcery. Given that situation, it is entirely understandable that the fathers thought what they did. But the fathers also endorsed slavery (with a few notable exceptions such as St. Gregory of Nyssa), and opposed usury.
One final note. We Orthodox who endorse the notion that there are serious circumstances in which one may use birth control, are not saying, "ra ra! yeah birth control! let's do it whenever we want!" All Orthodox are expected to keep the fasts which means no marital relations. And, the use of birth control is always less than ideal and only permitted by way of pastoral dispensation (oiconomia). Well, that's all I have to say.