I believe there are some flaws in your argument. If the Church’s directive that we must be open to life meant that the woman must be fertile, then we would all be mandated to chart the wife’s fertility and *none *of us would be allowed to have sex unless she was ovulating. (And, of course, wives who were infertile for any reason could never have sex). Perhaps you think that a couple who has sex during fertile periods should be given a bonus of getting to have sex during infertile periods, but I would like to see the theology behind that thought. Either it’s okay to have sex during infertile times or it’s not. The Church says it’s okay. I’ll stick with that.
That is why there’s nothing wrong with NFP. The couples are not trying to cheat and block conception while they enjoy sex during the fertile periods – they sacrifice and abstain. It’s not a sin to abstain.
Again, and again, and again, being open to life does not depend on what nature is doing with the couple’s bodies. It is entirely about the truth or lies that the couple tells with the actions of their bodies. To be truthful with your body is to keep the unitive and procreative aspects of sex in place. To lie with your body is to separate them. NFP couples do not separate them.
BTW – I am not an NFP user, so I am not trying to justify my own personal behavior. In fact, the only time I did use NFP was to try and get pregnant (didn’t work)!