"Natural" definition (lack of artificial.. anything) is very flawed when you consider biology, lack of a full understanding of the implications of the biology of reproduction slaughters it.
If you really want to be fully natural and believe that God made a cycle to control your sexual life, you should have sex only on fertile days. There is absolutely NOTHING natural about having sex with a woman when she is infertile, her whole body is preparing for another round and every single organ says "NOPE" to sex. Going against the nature but trying to convince yourself it's "natural", or using contraception like pills, it's all the same from biological point of view, i.e. tricking organism contrary to what it's purpose in this moment (pill actually less, since you have sex when body is ready for it, derp).
Intention is extremely important in Christianity, and artificial contraception in healthy marriages without superiority complex and views on sainthood before death (like some in this thread, not showing fingers
), will enhance mutual love, if used properly for spacing children or protecting health, not avoiding kids entirely. Sex in Orthodoxy, as I understand it, does not have many goals at once, it has many goals as they are necessary. Sometimes it is, like st. Paul said, to kill lust, the burning. Should we just keep lusting, watching other women (I won't even remind dat Jesus quote..) and let satan play with the mind while you wait for the right moment to actually release your lust (not quite)"naturally", just for the lust's sake instead of use what God provided and love your wife whether both of you want it.
Yes, I'm an archaeologist.