People have sex a lot. Every instance increases the chance of falling into the failure of efficacy risk. And, no contraceptive is 100%.

A 99.9% risk essentially covers 99.9% of sexual encounters. That 0.1% of times, those partners expected to be protected. Stuff goes wrong no matter how many precautions we take. You can do everything right and still find yourself flat.

Also, to that more than 50% who did everything responsibly, add all the victims of incest, violent rape, acquaintance rape (which is more frequent than you imagine), health reasons, etc. All that factored together could be argued to raise the percentage nearer two thirds of unwanted pregnancies.

When we place the onus on sexual choice, it risks the health, autonomy, liberty, and humanity of walking, breathing girls and women for the sake of a potentiality.