Theorem: Condoms are false.

Proof (by contradiction): Assume, for the sake of contradiction, that condoms are true. As we know, condoms are therefore made of latex and occasionally lubricated. At this point, we can see that the rest of the proof is obvious, thus we leave the details of the proof as an exercise to the reader. Thus we can see that our supposition is false. Therefore we must conclude that condoms are false.

Alternatively,

Proof (by divine intervention): Consider condoms. Condoms, as we all know, can be used as a form of birth control. The Pope says that birth control is against God's will. Thus, condoms are false.