Puritan Board Doctor
An example: having crops to farm as a farmer is a blessing but a farmer doesn't necessarily try to farm as much land as possible--he farms as much as he thinks he can.
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.
The anti-contraception lobby would contend that it is sinful to know one's wife if your intention is not always to try for a baby at the same time.
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.