The effectiveness of birth control is something that can be difficult to properly measure on many levels. Each form of birth control is only effective up to a certain percentage, with condoms being only effective 80% of the time and such. However, that is not taking into account various other factors that either the people making the chosen method of contraception or the people using it have direct control over. Regardless, the question regarding the effectiveness of birth control and which ones are known to be completely reliable will continue to be asked. That doesn't mean that there's ever going to be any concrete answers outside of the quasi-traditional “abstinence” response, but that's never stopped people from asking before. Chances are, it never really will.
Abstinence, when it comes to discussions of the effectiveness of birth control, takes the prime spot. You simply cannot get pregnant in a practical manner (as practical as pregnancy can be, at least) if you're not having sex. There are ways around this thanks to medical science, but those require the tacit consent of at least the female half of the equation. However, this is strictly coming off from the “raw material,” of sorts. Abstinence is guaranteed to prevent conception, but only if certain external factors do not come into play. The fact is, while conservative groups will proclaim that abstinence is the only real effective form of contraception, they are ignoring a number of critical points.
First and foremost, abstinence only works if you're actively practicing it. This should be plainly obvious, but the moment that a couple ceases to abstain is the moment pregnancy becomes a concrete possibility. It may seem like common sense that if you're committed to abstaining and you stop, you're risking pregnancy, but it apparently isn't. Couples who abstain then have sex without the intention of having children often forget about using birth control methods. Clearly, this puts them at risk of pregnancy, without even the partial protection that a condom or a birth control pill can afford them. This has often been attributed as a fault of abstinence-only education, though that is arguable.
It should be also noted that being effective is not the only factors that people keep in mind when selecting a form of contraception. Abstinence may be absolute in preventing both pregnancy and the transmission of a number of STDs, but if people don't have the discipline to stick to it, then it isn't worth a whole lot. This is particularly true for couples with raging hormones and not a whole lot of self-control, which some cynics point out is a lot more common than people would like to believe. Of course, some people have pointed out that abstinence-only education does not really factor in human behavior, and even when it does, the media is blamed for everything. While these are arguable points, the reality is that most abstinence-only educated couples take great risks whenever their discipline wavers.
A practical solution to the problem might involve discarding the notion of “abstinence-only or birth control-only,” because neither option is completely effective. From a purely practical standpoint, abstinence is still the only completely effective birth control, but couples should be aware that of what they can use in place of it should they ever want to stop abstaining.