You are quite skilful at selectively snipping bits of scripture out of context. Read the whole chapter.
First off, you avoid quoting the very next verse where Paul says But I say this as a concession, not as a commandment. He nowhere states it is a sin to not come together. That is your assumption.
Next, the limit for a time is not defined. Is it weeks, months, years? We know that Mary and Joseph did not come together for at least 9-months because of the virgin birth. Did that break Pauls concession? I doubt it, because the key phrase of the verse is come together again, that Satan tempt you not for your incontinency (or self-control per other translations). We know that Mary and Joseph stayed apart for at least 9-months, so we know they had self-control. It is quite possible they maintained self-control after His birth. Scripture doesnt say one way or the other.
In any case to say Mary and Joseph would sin by violating Pauls concession is absolutely wrong and figment of your imagination.
The absolute commandment is to be fruitful, which Joseph and Mary were in the most glorious fashion in bringing us the Savior. In many instances the Church allows for complete abstinence in marriage, when the fruit of the marriage is either already present or impossible. That is called Josephine marriage and is perfectly valid. An example would be a couple wishing to join monastic orders.
The commandment of submission to the husband remains valid, of course, but Mary would have violated it only if Joseph had asked her physical affection, of which we have no record.
In any event, the uniquely exceptional nature of the Holy Family renders all attempts to find a legalistic fault in them stupid and vaguely blasphemous.
The Bible says that Paul says that WITH PERMISSION.
Because Paul wrote it does it make it any less valid?
Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness...
Also, Paul adds that he wishes all men were as he, celibate, but he knows this can not be the case for most.
1 Corinthians 7:5 Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
6 But I speak this by permission, and not of commandment.
7 For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that.
Married couples were only to come apart for a season. And he mentions a time of fasting and prayer.
No one except Jesus fasted longer than 40 days, so I expect that Mary and Joseph, if they abstained, it would never of been longer than 40 days.
The Bible says Joseph knew her in the sexual sense, and that Jesus had brothers and sisters.
deal.