Marriage, in the scientific sense, is sexual intercourse. Quite literally, that is what marriage is. When one thing marries with another, it becomes one new thing. That is marriage.
Marriage is becoming one with someone.
Jesus himself says that man is "joined to his wife; and they become one flesh".
That is the function of sexual intercourse. To become joined as one organism. To marry with another being physically, psychologically and spiritually. It is the ultimate expression, fulfilment and completion of love.
That is why sexual intercourse is never taken trivially within Christianity, but always treated with the utmost reverence and respect. Because it is an irreversible, binding act. This is also why promiscuity, adultery and divorce are viewed as such extreme tragedies - because once you marry with someone, you are bound in such a deep, complex way, and tearing that asunder is quite literally the severing or chopping up of the one organism.
The marriage ceremony is one of the great mysteries or sacraments of the church. It exists to express the significance of what is about to be undertaken, and give power and direction to the couple, and to acknowledge God's part in all of it - to remind the couple of agape, or Christ's love - and to reflect this in their relationship.
In fact, the marriage relationship, in the Christian worldview, is actually a relationship of three - a trinitarian relationship of man, woman and God. A perfect mirror of the Holy Trinity itself! Incredible. Without God in the picture, something is lacking.