Does St Paul command us to use contraception?! Part I

The other night I heard an interesting objection to  Natural Family Planning for the first time. Its novelty intrigued me, so I've been looking into it a bit. Here's essentially how it went:

St Paul tells us in 1 Cor. 7:

"Now concerning the matters about which you wrote. It is well for a man not to touch a woman. But because of the temptation to immorality, each man should have his own wife and each woman her own husband. The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not rule over her own body, but the husband does; likewise the husband does not rule over his own body, but the wife does. Do not refuse one another except perhaps by agreement for a season, that you may devote yourselves to prayer; but then come together again, lest Satan tempt you through lack of self-control." (1 Cor. 7:1-5)

So, the only reason a married couple may abstain from sexual relations is for prayer. Since NFP requires periods of abstinence when the woman is fertile (a period of about 10 days per month?) if a couple is seeking to space children for grave reasons, it violates this command. The only way to both not violate this command and to space children, the argument goes, is to use contraception, especially during times of fertility. Therefore, St Paul is effectively commanding married couples to make use of contraception, while forbidding NFP.

Radical.

When I was in the discussion at the time, I didn't think to open up my bible to the passage in question. If only I had. I could have cleared up one little matter that is mystifying me.

The fact is, the very next verse seems to me to undermine the notion that it is a command. Check it out:

 "I say this by way of concession, not of command." (1 Cor. 7:6)

Paul himself seems to be saying that this is not a command! I think this blows quite large holes in the argument, which relies on the fact that it is a direct biblical command that must not be violated.

Stay tuned for Part II!! (I had to split it up, as it was really getting far too long.)