(Tim: under an earlier post, I responded to a dear brother who asked the same question we all have--namely, isn't being fruitful and multiplying more an Old Testament than a New Testatment command?)
Dear Brother, don't be dismayed. About 99.999% of Reformed officers in America today--in fact, 99.99% of any Reformed officers since Margaret Sanger (founder of Planned Parenthood) engaged in civil disobedience and got the Supreme Court of these United States to reverse our nation's Comstock laws last century--have believed what you articulate. Here's a more crass way of puting it:
We've been fruitful, so let go of this Old Testament patriarchal take-woman-into-the cave-and-have-your-way-with-her neanderthal mindset. It's so demeaning to women. Haven't they suffered enough already? Do they have to spend their lives at home making babies, cooking, and changing diapers? Would any servant leader do that to his wife?
Please don't be offended. I know this is not how you put it. But having known and loved many Reformed officers over the years, this is a pretty accurate summary of the state of our obedience. We've evolved. We've learned scientific truths the Reformers didn't know. We need to focus on the quality--not the quantity--of our childrearing. We need to educate our daughters as well as our sons, and give them a chance to live...
But putting aside the particular nuances of this or that man's thoughts on birth control, we must face the fact that the Reformers didn't agree.
Rather, as the Church speaks with one voice condemning rebellion against other Order of Creation laws instituted by God such as life-long, monogamous, heterosexual, patriarchal marriage, she also speaks with one voice concerning this other Order of Creation law that obstructing fruitfulness in the marriage bed is sin.
That's what wounds my conscience when I simply tell my congregation that fruitfulness is a blessing that should be welcomed and every decision to use birth control is a spiritual decision--never simply a pragmatic one. To most, even that sounds monstrous. But to me, even that sounds cowardly. This voice keeps awakening my conscience to the fact that, prior to 1950, Protestants were as opposed to birth control as Roman Catholics.
And as I said to a dear elder brother of mine who thinks even what I teach creates unreasonable difficulties for our young couples, what do we do with the universal understanding of the Church? Just kiss it off?
Someone help me, brothers. How can we say, let alone believe, that just at the time when the Western world threw out every other Order of Creation truth concerning God's gift of sexuality, true reform hit us in birth control and we learned we were always wrong?

