It is a sacred symbol.
Next, Elder Holland outlines how the act of intimacy stands as a symbol—of two combining to become one—an act so beautiful it is only acceptable under sacred and proper circumstances. He says:
May I suggest that human intimacy, that sacred, physical union ordained of God for a married couple, deals with a symbol that demands special sanctity. Such an act of love between a man and a woman is--or certainly was ordained to be--a symbol of total union: union of their hearts, their hopes, their lives, their love, their family, their future, their everything. It is a symbol that we try to suggest in the temple with a word like seal. The Prophet Joseph Smith once said we perhaps ought to render such a sacred bond as "welding"--that those united in matrimony and eternal families are "welded" together, inseparable if you will, to withstand the temptations of the adversary and the afflictions of mortality. (See D&C 128:18.)
But such a total, virtually unbreakable union, such an unyielding commitment between a man and a woman, can only come with the proximity and permanence afforded in a marriage covenant, with the union of all that they possess--their very hearts and minds, all their days and all their dreams. They work together, they cry together, they enjoy Brahms and Beethoven and breakfast together, they sacrifice and save and live together for all the abundance that such a totally intimate life provides such a couple. And the external symbol of that union, the physical manifestation of what is a far deeper spiritual and metaphysical bonding, is the physical blending that is part of--indeed, a most beautiful and gratifying expression of--that larger, more complete union of eternal purpose and promise.
As delicate as it is to mention in such a setting, I nevertheless trust your maturity to understand that physiologically we are created as men and women to fit together in such a union. In this ultimate physical expression of one man and one woman they are as nearly and as literally "one" as two separate physical bodies can ever be. It is in that act of ultimate physical intimacy we most nearly fulfill the commandment of the Lord given to Adam and Eve, living symbols for all married couples, when he invited them to cleave unto one another only, and thus become "one flesh" (Genesis 2:24).
To own a copy of this powerful talk for yourself, check out Of Souls, Symbols, and Sacraments atdeseretbook.com. Or, for additional reading on sexual intimacy in marriage that is in-depth, frank, open, and uplifting, check out And They Were Not Ashamed and The Act of Marriage.