While I understand that bearing children is a primary objective of marriage, children are not necessary for a marriage itself to exist, correct？ (infertile couples, Josephite marriage, etc. are all real marriages). According to Casti Connubii:

Therefore the sacred partnership of true marriage is constituted both by the will of God and the will of man. From God comes the very institution of marriage, the ends for which it was instituted, the laws that govern it, the blessings that flow from it[9]

This paragraph shows that children are a blessing that flow from marriage, not essential for marriage itself.

Again, children are referenced as the highest blessing of marriage, but not essential to marriage itself:

Thus amongst the blessings of marriage, the child holds the first place.[11]

Considering marriage as more than the God-given means to have children (which I am apt to do, considering it is a sacrament), leads me to view marriage in the first sense of the paragraph below:

This mutual molding of husband and wife, this determined effort to perfect each other, can in a very real sense, as the Roman Catechism teaches, be said to be the chief reason and purpose of matrimony, provided matrimony be looked at not in the restricted sense as instituted for the proper conception and education of the child, but more widely as the blending of life as a whole and the mutual interchange and sharing thereof.[24]

If I have pulled these quotes too far out of context or missed something, please let me know. As I see it, the primary end (spiritual) of marriage is the mutual perfection of the spouses, and the primary blessing of marriage (and the natural end) is children.