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David: Your critique of Capps’s argument (on the succinct formulation of it you present) is, I think, correct. The conclusion validly derivable from premises 1. and 2. would be something like “the conjugal act is the more permissible kind of sexual act” which is not the conclusion Capps would like to reach. (To be absolutely certain, we should translate the argument into proper logical form but, for the moment, we can consider it more informally.)
But even if the argument as stated were valid, its soundness would be a matter of dispute for, as you correctly note, premise 1 is highly problematic. Most moral theories distinguish between our moral duties and acts of supererogation. So, for example, we might reasonably be taken to have a moral (not legal) duty to render assistance to people in dire need where our efforts involve little or no risk or expense to us, let’s say by throwing a lifebuoy to someone in danger of drowning. But it would be a different matter to say we have a moral duty to dive into a torrent to effect such a rescue. It might be admirable and commendable so to do but failure to do so could hardly be regarded as a failure to carry out one’s moral duty.