You can't really mean this, since frustration of the nature and purpose of the conjugal act is intrinsically evil, per Pius XI, and therefore can never be permitted.
Rather, not having children by use of NFP is problematic because of the failure to fulfill the duty owed to society of propagating the human race. This has to be separated from the malice involved in contraception, just as Pius XII does:
The matrimonial contract, which confers on the married couple the right to satisfy the inclination of nature, constitutes them in a state of life, namely, the matrimonial state. Now, on married couples, who make use of the specific act of their state, nature and the Creator impose the function of providing for the preservation of mankind. This is the characteristic service which gives rise to the peculiar value of their state, the bonum prolis. The individual and society, the people and the State, the Church itself, depend for their existence, in the order established by God, on fruitful marriages. Therefore, to embrace the matrimonial state, to use continually the faculty proper to such a state and lawful only therein, and, at the same time, to avoid its primary duty without a grave reason, would be a sin against the very nature of married life.
Otherwise your reasoning will be unable to distinguish at all between the practice of periodic continence and the use of contraception, leading either to approval of both or the condemnation of both, conclusions which are ruled out for us by the authority of the Church.
frustration of the nature and purpose of the conjugal act is intrinsically evil, per Pius XI, and therefore can never be permitted
You make a valid point here, that I should have distinguished between "nature" and "purpose." Artificial contraception violates the "nature" of the conjugal act, which is always and in every case wrong. Periodic continence may violate the "purpose" of the conjugal act, depending on the motives of the participants.
Otherwise your reasoning will be unable to distinguish at all between the practice of periodic continence and the use of contraception, leading either to approval of both or the condemnation of both, conclusions which are ruled out for us by the authority of the Church.
One is condemned in all cases, the other is approved only in exceptional circumstances. I believe that the comparison to missing Mass on Sunday is apt. What were to happen if the exceptions which justify missing Mass on Sunday were extended to a universal approval, and it was even praised as a positive good? That would be analogous to the current situation in which there is a culture of NFP propaganda.