My wife and I are certified NFP instructors with the Couple to Couple League. You are misleading with your statement
"The methods of natural family planning (such as rhythm and basal body temperature) are less likely to be as effective as the hormonal methods of birth control".
Rhythm is not modern NFP. In fact very few people use that outdated system. I know of no one. Modern methods of NFP are scientifically proven to be as effective or more effective at postponing pregnancy as any type of contraceptive. "A study completed in Germany in 2007 found that the symptothermal method has a method effectiveness of 99.6%."
Here it is if you want to read it for your self: http://humrep.oxfordjournals.org/content/early/2007/02/20/humrep.dem003.full.pdf
So yes NFP can be very effective for all women and it has no negative side effects and cost nothing once you learn the method.