If we are to assume a futture with to little oil to maufacture condoms or synthesise "the pill", i would suggest a partial solution may be found in carefull and medically validated use of herbal contraceptives.

From the inital research i have done there seems to be real merit in female consumption of "Queen Anne's Lace" seed within 8 hours of intercourse, and then taken daily for some time after as a means of preventing conception.

Of Equal value (herbaly speaking)would appear to be Neem Oil taken by males causes tempory sterilisation that has no lasting effects and is reversed completely within 6 months of ceasing to take the daily dose, the only apparent arguable downside of Neem is that it lowers libido as well.

Im not saying that these are a perfect solution, but they are capable of being sustainable (imho), even in a low energy environment.

Combined with a better knowledge of fertile and non-fertile periods of a womans cycle (vatican roulette) birth controll is probably relatively replicable, just without the ease and flippancy that it can be achieved today.