Just to make sure we have them before us, these are the documents that signalled the change (not Lambeth 1930) from the patristics that HV implicitely (but not explicitely) is based on, as far as HV is based on patristics at all.
The first time Rome spoke on the matter was as long ago as 1853, when the Sacred Penitentiary answered a dubium (a formal request for an official clarification) submitted by the Bishop of Amiens, France. He asked, "Should those spouses be reprehended who make use of marriage only on those days when (in the opinion of some doctors) conception is impossible?" The Vatican reply was, "After mature examination, we have decided that such spouses should not be disturbed [or disquieted], provided they do nothing that impedes generation" By the expression "impedes generation", it is obvious the Vatican meant the use of onanism (or coitus interruptus, now popularly called 'withdrawal'), condoms, etc. For otherwise the reply would be self-contradictory and make no sense.
"Qu:. An licitus sit usus matrimonii illis tantum diebus, quibus difficilior est conceptio?
[Whether it is licit to make use of marriage only on those days when it is more difficult for conception to occur?"]
"Resp.: Coniuges praedicto modo utentes inquietandos non esse, posseque confessarium sententiam de qua agitur, illis coniugibus, caute tamen, insinuare, quos alia ratione a detestabili onanismi crimine abducere frustra tentaverit" (DS 3148).
[Spouses using the aforesaid method are not to be disturbed; and a confessor may, with due caution, suggest this proposal to spouses, if his other attempts to lead them away from the detestable crime of onanism have proved fruitless.]
This decision was published in Nouvelle Revue Théologique, vol. 13 (1881), pp. 459-460, and then in Analecta Iuris Pontificii, vol. 22 (1883), p. 249.
"De uso exclusivo temporum agenneseos:
[Regarding the Exclusive Use of the Infertile Period]
"Qu.:An licita in se sit praxis coniugum, qui, cum ob iustas et graves causas prolem honesto modo evitare malint, ex mutuo consensu et motivo honesto a matrimonio utendo abstinent praeterquam diebus, quibus secundum quorundam recentiorum theoremata ob rationes naturales conceptio haberi non potest?
[Whether the practice is licit in itself by which spouses who, for just and grave causes, wish to avoid offspring in a morally upright way, abstain from the use of marriage-by mutual consent and with upright motives-except on those days which, according to certain recent [medical] theories, conception is impossible for natural reasons]
"Resp.: Provisum est per Resp. S. Paenitentiariae, 16. Iun. 1880."
[Provided for by the Response of the Sacred Penitentiary of June 16, 1880]http://www.freerepublic.com/focus/f-religion/1165258/posts#FN_6
On the use of natural law, it is used in Orthodoxy, as much as canon law is used in US law:only to illuminate the issues for resolution according to other principles (in the case of the US law, the constitution and common law;in the case of Orthodoxy, revelation). It does not, nor should it for Orthodox, form the basis of the moral order.