The unitive purpose of marriage only loses its purpose in the face of infertility if you assume that the fallen nature of mankind is the way that it was meant to be. If you take a more Theology of the Body approach to things and look at Jesus' words - "In the beginning, it was not so" then you would see why I worded my answer the way that I did. We weren't made to be infertile by God's original intent. Original sin did that and now we have to live with the consequences. Even an infertile couple is still called to be life-giving, even if that life is through ministry or through adoption. Again, I can offer you a full natural law argument behind the dual purpose of marriage, but it's lengthy so I didn't want to type it out without invitation.
And I'm aware that in our Eastern brothers are allowed to participate in both marriage and Holy Orders but even there, there is a particular order to the sacraments. You must marry before receiving Holy Orders and once you are ordained, you do not marry again. The same goes for deacons in the Western Church. And in the Eastern Church, a married priest never becomes a bishop, only a man who has chosen to enter the priesthood celibate. There are, of course, a variety of pastoral reasons for that, but one of them is that higher sacramental sign of Christ's marriage to the Church.
I think it's unfair to say that marriage is simply about two people trying to draw each other close to God. Any two friends can and should do that. Married couples are called to do that in a different way precisely because they create families between them. There is a reason that Paul compares the relationship between Christ and His Church to a bridegroom and his bride in Ephesians 5, not simply two best friends. To draw on your example, I'm sure we can all pick out two people in our lives who have been life-long friends and also two people who divorced. So why did Paul - and thus the Holy Spirit - pick the example that he did? There is something different about married love perfected - remember that we live in a fallen world - than simply brotherly love perfected. It is the fact that the two become one flesh, that they form families and live out their lives for each other regardless of where they came from before and that new life comes from that union - something unique to marriage that cannot be replicated in friendship, no matter how strong. That is supposed to tell us something about Christ and His Church that no other relationship can.