There is one proper vehicle for sexual activity. It is called marriage. And Christians vigorously defend that divine institution.
When God created the world He said, "It is not good that the man [Adam] should be alone." (Genesis 2:18) So God creates, from Adam's side, Eve, "a helper fit for him." This is the first marriage.
("Helper" is also a term used for God elsewhere in the Bible, such as Deuteronomy 33:7 and Psalm 33:20, so it obviously is not meant to give the woman/Eve a second class status. If anything it means quite the opposite.)
Thus we have the foundation of the Christian belief that marriage should consist in the life-long bond of one man and one woman to love and cherish each other.
One man is not permitted to take several wives, although many in Scripture sin in this and incur God's judgment. One woman is not permitted to have several husbands. A man cannot take a husband. A woman cannot take a wife. It simply does not work.
One reason we stand so vigorously for this institution is that Saint Paul tells us that they relationship of mutual love and respect between a man and a woman joined in marriage gives us a glimpse and is patterned after the relationship between Jesus Christ and His bride, the Church.
Christ acts as the bridegroom, the husband. He gives up His very life for His wife. He thinks only of what He can do to "present the Church in splendor". (Ephesians 5:27) He gives love repeatedly to His bride.
The bride, the Church, then submits (follows the lead of) Jesus Christ. It shows Him the respect He deserves because of the love He has shown in the cross.
Therefore, when a husband loves his wife (imperfectly, failing and falling) he gives his wife, and the world around them, a glimpse of the love that Jesus Christ has for His Church.
When a wife respects her husband's leadership and follows his lead (imperfectly, failing and falling) she gives the world a glimpse of the kind of respect that the Church owes to Christ.
You cannot have two Christs. You cannot have two Churches. There is only one of each. Marriage should reflect that as best it can.
The union of Christ and His bride will stand for eternity. Let us stand for marriage here in time.
[Tomorrow: We stand for The Mass.]