And where in scripture is the farmer commanded to farm all his land and not defraud his fields of seed? Nowhere. And where is man commanded to know his wife and not defraud her of him nor vice versa? In 1 Cor 7. I think you mistake their meaning (though I don't mean to speak for all of them). They contend that it is sinful to derail the natural effects of knowing one's wife, not that you must try to procreate at all times. Motive is the key.