Any form of birth control is messing with God's plan. There should only be one form and that's abstinence!
That is interfering with God's plan as well is it not? To think that you can determine when and if you will have children in your marriage? It works great for single folks, but for married folks, it is in violation of Biblical mandates.
If you have a proper Biblical relationship with your spouse, you will not use this method either since it is more clearly unscriptural than any other method of BC. There are only 2 reasons to abstain that can be scripturally supported, and avoiding pregnancy is not one of them.
Do what you will regarding Birth Control, its a matter between you your spouse and God, and he will still determine when and if you have your children. Plenty of folks get pregnant on BC, whatever method, and plenty who aren't, don't.
There are a ton of considerations regarding how many children to have, lets look at a few.
1. Have a full quiver.
Being somewhat of a dispensational thinker when it comes to theological organization, you have to view what the original author (divine and human) had in mind when they wrote that. In the OT, the blessing was tied to the land, and the fruit of it, plus the blessing was tied to your ancestors. Not so much today, as our blessings are less tangible in nature, and not as physically specific. The fruit of the Spirit, and our spiritual blessings in Christ Jesus are not in any way tied to the number of children we have.
2. Interefering with God's Plan -- see above, almost everyone I have heard preach this, also advocates the rythym method (you do know what they call folks who use this method right? Pregnant)
3. I can't afford more than x number of kids -- What children God chooses to send your way, he will help you take care of, but that doesn't mitigate our responsibility for proper family planning and stewardship of our resources.
Unfortunately, I have also seen this doctrine taught heavily in many homeschool curriculums as well as the Bill Gothard Garbage. Maybe its a way for them to generate more revenue, but that might just be a little too cynical on my part.