Water can Wet You, and Fire can Burn

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A learned reader asks that since technology reduces the risk of pregnancy, the influence of conception-morality on sex-morality is lessened, why do I conclude that  sex-morality  can nonetheless be completely deduced from conception-morality?

He asks for clarification, which I would be glad to provide, if I could. I fear my powers of description, in the limited space here, are unequal to the task. Let me at least offer a summation of the argument, which can be, as...