Newatthis The advice to go out and multiply is an Old Testament command, not a New Testament comand and Christianity is based on the New Testament. it looks back to the Old Testament, it respects it and sees it as prophesying what was to come, but it is not the basis for Christianity.
The decision that priests could not marry was made in the mid 12th century at a time when being a priest was a far more rigourous task than it is now. A priest is expected to devote his whole life to his service as a priest and it was considered that could not happen if he also had family responsibilities.
In the past priests would be serving huge areas and constantly travelling and while priests do not take vows of poverty, as monks do, they are expected to live very simply, especially when they are working with parishioners who are themselves living in poverty. Catholic priests do not receive a minimum salary as many do in other religions and must depend on his parishioners for his stipend, which in poorer parishes may be very small. It is one thing to dedicate oneself to a life of poverty, when you could have a career that was better paid but to expect your wife and child to do so also, is, perhaps a sacrifice too far.
There is nothing in principle to stop the church changing its policy on this - and I suspect this will become needful as vocations fall. Already married men with careers and families are entering the preliminary levels of being priests by becoming deacons, but their service is essentially part time and one day we may have to accept the end of celibacy and having a part time priesthood.