Catholic marriage is not supposed to be a "symbol" of anything. It is supposed to be a real covenant in which a man and a woman give themselves completely to each other in a reflection of the love between the Father and the Son, and between Christ and his Church.
A child is the result of that love.
In my work I have met many men who told me how they had to marry their wives becasue of the pregnancy. They felt used.
I always thought that the time a young couple spends together prior to the children arriving as the cement to help lay a strong foundation in the marriage.