Sword, I want you to know that I do understand what you are saying, and I agree that the primary purpose for sex was to have children, but, as with divorce, there are certain laws which were changed. In the OT, some foods were not to be consumed. Now, all food can be consumed without fear of sinning. Divorce was, at one time, completely unacceptable, until it was allowed under specified circumstances. As with these, it was necessary for God to make the frequency of sexual intercourse subject to the will of a man, and his wife, hence, Paul's letter to the Corinthians.
I am also inclined to believe that this change occured because of man's inability toward self discipline, and not because the primary purpose of sex was for pleasure. Some would say that sexual desire is from God. This is only partly true. The ability to procreate is certainly accompanied by a needed ability for arousal. That said, sexual desire is necessary before and during the sexual act, however, during the course of a day, there are many factors which increase and decrease sexual desire, which simply produce, and store, lustful thoughts, this aspect of desire is not from God, and yet has the propensity to enhance a man's desire to be sexual with his wife.
I believe that for this reason, Paul addressed this by necessity as a way for both partners to relieve themselves of stored sexual tension, which, if left unattended, could result in adulterous activity resulting from temptation.
For this reason, I'm inclined to agree with you. I do believe that the original purpose for sex was procreation, but because of man's nature and his inability to consistently abstain, permission was given to married couples to have sexual intercourse at will, while others must rely on God's grace to help them to abstain from all types of sexual activity, but those who are married and are engaging in sexual intercourse at will, are not sinning.
Would you agree with this?