If sex is strictly for procreation, it shouldn't be pleasurable.

Thesis: If God intended for sex to strictly be for procreation in the context of marriage, he shot himself in the foot by making it pleasurable.

If sex were not pleasurable, dutiful Christian couples would still procreate out of obedience. Non-Christian, non-heterosexual, and/or non-married couples would be far less likely to have sex.

There would ostensibly be many benefits to this approach.

• Christians would out-breed non-Christians, resulting in more Christians.
• There would be more nuclear families and less risk of disease.
• Less people would be tempted to sin.

However, God instead created extraneous biological systems that make sex tremendous fun regardless of the context, working against his own ends and creating all the problems abstinence advocates rail against.