No contraceptive method is 100% effective.

hence "can be"

We don’t need airbags because injury can be prevented by seatbelts.

The "don't need" part is a fictitious part you inserted into the argument and not a claim I made.

Injury "can be" prevented by seatbelts, that is a valid claim.

I don't know what else you could have meant when you answered the question "Why not both" with "one can be prevented by the other".

The implied meaning is: "Not both because one can be prevented by the other".

Hence "both".

not prevented… (trust me :)) but best we can do!!