Tells a lot here. http://home.teleport.com/~packham/temples.htm

As lozboz said, it’s certainly not secret; it’s sacred. Most temple weddings include the sealing ordinance, which allows the marriage to be in effect not just for this life, but forever. The marriage is performed as the husband and wife kneel at the altar, across from each other, and promise themselves to each other. It’s really not that different from most non-temple weddings, with a few exceptions: 1) There’s no walking down the aisle; the couple simply kneel at the altar. 2) Nobody gives the bride away; she gives herself to her husband, just as he gives himself to her. 3) There’s no invitation for individuals to object. If they were going to object, they should have done it before that point. ;-) 4) I know I mentioned this before, but this is by far the most important difference: it’s not generally “till death do you part” nor “as long as you both shall live”; it’s “for time and all eternity.” I suppose another important difference is that, since the sealing ordinance builds upon the covenants made through other temple ordinances (e.g. the endowment), only those that have received said ordinances are invited to a sealing. Again, this isn’t because we’re trying to keep anything secret; it’s just that we don’t want people to wind up being accountable for things they’re not ready for, just yet. We’re each at different points in our spiritual progression, and it would be silly to think that any of us are ready to comprehend all the things of God. That’s why He teaches us line upon line, precept upon precept, until some day in the far, far distant future, we finally receive the fulness of all that He has to give. If you’d like to know more about the temple, here’s some great resources: • http://tinyurl.com/holytemple • http://mormon.org/temples HTH!

Secret because the "Union" used to be "Consummated" right there. This was only for the Highest "Officials" of the cult (Big Bucks Families). This I think was stopped when Govt. got involved in the "Polygamy" aspect of their group. John

It is secret, because if a perspective convert were to know what happens they would see the lies, evil and occult practice it is.