It isn't secrecy. The Church of Jesus Christ of Latter-day Saints holds the Temples and the ordinances there to be sacred. So sacred that even worthy, Temple-attending members are not even supposed to discuss the details amongst themselves outside of the Temple's confines. I'm sorry if you can't understand how people can hold a place or certain things to be so holy, or sacred, that they should be protected from things or activities which would defile them.
Non-Mormon parents can’t attend their child’s Temple wedding?
That’s creepy.
If you didn't know it, most of the "secret" and "holy" stuff in the Temples was copied from the Masonic rituals. Real holy stuff. Google it.