Natural family planning is different from the rhythm method, and it is not just guessing about when ovulation will occur. The natural family planning method requires a woman to monitor her body temperature every day of every month and matching it to a chart to determine when she is ovulating and then avoid intercourse during that time. And that's just the easiest way to do it. To really ensure that ovulation is occurring when predicted, the woman should also be comparing her cervical mucus viscosity (stickiness), to viscosity and temperature charts daily. In order to get the 75-87% effectiveness that Planned Parenthood predicts, the methods must be used correctly and consistently. This number drops considerably when only one measurement is used or the couple isn't really diligent about following the woman's temperature or cervical mucus.
The CDC just released results of a survey that said that more teens have been using the rhythm method--up 11% since 2002. If these teens are using the rhythm method IN ADDITION TO OTHER FORMS OF BIRTH CONTROL, that's great! Avoiding the most fertile times of the month, even when using condoms or other barrier methods is a great way to avoid pregnancy. However, if these young women are not measuring their body temperature and/or cervical mucus every day, but instead are just "guesstimating" when their most fertile period is, then this method is not really giving them the effectiveness they might be assuming. Many people confuse the rhythm method and natural family planning, which may lead to false assumptions about effectiveness as preventing pregnancy. Natural family planning takes a lot of work and knowledge about to be effective. If also will not protect partners from STI transmission.
All things considered, there are many options that are easier to use and are much more effective. Birth control pills take the ovulation out of the equation for you, so you don't have to measure any fertile period, and condoms both protect against fertilization and STIs when used correctly. Taking a pill every day is much less of a hassle than recording your body temp and cervical mucus data and comparing it to a chart. You can get condoms for free at any Planned Parenthood health center. Make sure to follow the directions!! They don't have to be a burden in the bedroom, either. If you think a condom as protecting yourself and your partner, and both of you help in the application process, condoms can be a pretty sexy addition to foreplay!