In the same way, although cooking doesn't always make food safe to eat, there is a form of cooking that's simple and 100% effective: not eating.
And although the various safety measures rock climbers use sometimes fail, there is a method of rock-climbing safety that's 100% effective: not going rock-climbing in the first place.
And I have a perfectly safe car: that is, I never leave my house.
I take it you can see how ridiculous these are. The point (well, one point) of cooking is to be able to eat food and not get food poisoning. The point of all the ropes and things is to be able to climb rocks and not fall to your death. The point of seatbelts and crumple zones and antilock braking is to be able to drive from place to place and not get killed in a car crash.
Not having sex -- which I assume is what you are referring to here -- isn't a form of birth control, because it doesn't accomplish the objective of having sex and not making babies.
It might still be a good idea, of course. Just as it might be a good idea never to go rock climbing (maybe it's just too dangerous to be really worth it for anyone who's thinking clearly) or never to drive a car (it's bad for the environment in lots of ways, after all). But that's a separate argument, and the case needs to be made honestly (cost/benefit on failure rates, appeals to alleged rules handed down by alleged gods, etc.) rather than by snarky one-liners like the one above.