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.
No, it doesn't. Learn to read.
Concession or permission, depending on which version of the Bible you reference. Either is acceptable for the Greek, if you check your Strong's. The important point is that it is not by commandment.
Because Paul wrote it does it make it any less valid?
No, it's scripture, so it is valid. It's just not a commandment like you try to make it out to be.
I expect that Mary and Joseph, if they abstained, it would never of been longer than 40 days
Well, you are wrong already. Mary and Joseph abstained throughout Mary's pregnancy and Jesus was born of a virgin. Also, you've not proved through scripture that if they abstained longer than 40 days it was a sin. You can speculate all you want but you haven't proven anything.
The Bible says Joseph knew her in the sexual sense
No, it does not. You've been asked to show where this is stated in scripture, but you can't because it's not there. The scripture is silent on this point. All you are doing is making some assumptions.
Jesus had brothers and sisters
Where does scripture say these brothers and sisters of Jesus were born of Mary? Post scripture, not your speculation.