Contraception is not right if it violates the purpose of sex or obviates valuable life. Some acts of contraception obviate valuable life. If a thing is not right, then it is wrong. Hence, some acts of contraception are wrong. (Cx: x is an act of contraception; Rx: x is right; Vx: x violates the purpose of sex; Ox: x obviates valuable life; Wx: x is wrong)