The Sacrament of Baptism is often called "The door of the Church" because it is considered the first of the seven sacraments. According to Catholic Catechism (1212), the Catholic Church sees baptism as the first and basic sacrament of Christian initiation. For most Christians and especially Catholics, baptism is the first sacrament received (often as an infant) after which one becomes a member of the church. Baptism is also required by the Catholic church in order to receive the other sacraments. For example, the Sacrament of Confirmation and the Sacrament of Holy Communion depend on first being baptised.
For such a couple, it would be required by the Catholic Church for baptism to take place before being married otherwise the marriage would not be considered a sacramental marriage, and the church would not be allowed to perform such a marriage.
The Sacrament of Marriage is built on a firm foundation of Baptism. Clearly from the Catholic point of view, marriage cannot take place before both man and woman are baptised. The Catechism (1633) describes a mixed marriage as between Catholic and a baptised non-Catholic that requires particular attention on the part of couples and their pastors.
It further states "marriage between a Catholic and a non-baptized person requires even greater circumspection" meaning a marriage to a non-baptized person would create complications and at least raise some tough questions about one's faith. There is a passage in the Bible in 1 Cor 7:14 about being married to an unbelieving spouse and the believing spouse a blessing to the other so there are some allowances for "mixed" marriages, but that is the exception not the rule.
Without a baptism, the commitment of the couple to the faith and church would be in question. When children come into the picture then it becomes even more important to decide whether one is a Christian or not and whether children should be raised as Catholics or not. The Bible clearly talks about "luke warm" Christians (Rev 3:16) and rebukes such attitudes. From the church's perspective, this is very relevant since you next have to ask about the commitment the couple will have to one another.
Of course, a secular stand point would have an entirely different point of view on this topic, but the context here is between two "nominal" Catholics who would like to get married in the church.