Q. 1. What is the purpose of marriage in the Catholic Church? Many of my friends do not want children. They are more concerned with making money and travelling around the world with no intention of ever settling down.
A. 1. To answer your question, the Catechism of the Catholic Church states:
# 1652 "By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory." [GS 48 § 1; 50]
"Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: "It is not good that man should be alone," and "from the beginning [he] made them male and female"; wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: "Be fruitful and multiply." Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing the spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day." [GS 50 § 1; cf. Gen 2:18; Mt 19:4; Gen 1:28] 
The primary purpose of the Sacrament of Marriage in the Catholic Church is the mutual love of spouses, such leading to procreation.
In view of the fact that the purpose of the Sacrament of Marriage is to procreate as commanded by God (Bible: Book of Genesis), those who marry in the Catholic Church without being open to the gift of life and the bringing up of children, their marriage is not valid.