In the Bible, the scriptural reason for getting married is to portray the image of Christ and the Church through the marriage. Ephesians 5:22-33 gives not only the reason, but the order of marriage within the church.
Wives, submit to your own husbands, as to the Lord. For the husband is
the head of the wife even as Christ is the head of the church, his
body, and is himself its Savior. Now as the church submits to Christ,
so also wives should submit in everything to their husbands.
Husbands, love your wives, as Christ loved the church and gave himself
up for her, that he might sanctify her, having cleansed her by the
washing of water with the word, so that he might present the church to
himself in splendor, without spot or wrinkle or any such thing, that
she might be holy and without blemish. In the same way husbands
should love their wives as their own bodies. He who loves his wife
loves himself. For no one ever hated his own flesh, but nourishes and
cherishes it, just as Christ does the church, because we are members
of his body. “Therefore a man shall leave his father and mother and
hold fast to his wife, and the two shall become one flesh.” This
mystery is profound, and I am saying that it refers to Christ and the
church. However, let each one of you love his wife as himself, and let
the wife see that she respects her husband.
So here's the scripture on husbands and wives, and it clearly lays out why marriage exists: Marriage itself is intended to portray the relationship of Christ to the church.
Opening that up a little more, we say that a husband should love his wife as Christ loved the church. So in that sense, the husband is a type of Christ, that is, he represents the picture of Christ within the marriage in the way that he selflessly loves his wife. This doesn't mean he himself is a messiah or savior, it simply means that his role is congruous to the role that Christ plays in respect to the church, mainly by virtue of self-sacrificial love.
Countering that point, the wife is to submit to and respect her husband. Recognizing him as leader within the marriage (and mind you, he ought to be a good leader... see the exhortation to the husband), she is to submit to him and respect him for the love that he selflessly offers her. Her role is to represent the church in this image, and to portray, in the marriage, the Church's response to Christ.
So marriage is to be an image of Christ and the church, with the husband representing Christ's love for the church, and the wife representing the church's response to Christ.
A Brief Aside on Homosexuality's Compatibility with Biblical Marriage
In her case, I'm not exactly sure that you can separate out the fact that she's a lesbian from the Biblical backing for why she would want to get married in the church.
It's not that two people of the same sex cannot be fond of one another, and that they can't even have sacrificial love towards one another. They most certainly can. The problem lies with the roles in the relationship. They cannot clearly portray the image of Christ and the church, because there is no clarity regarding whose role is whose. In other words, who, in this picture, is Christ, and who is the church?
The Biblical purpose intended for marriage cannot be supported in a homosexual relationship, because the Biblical purpose of marriage is to represent Christ and the church. The specific roles assigned to the male and female within the institution are what make this possible.