1) Temples are considered Houses of the Lord, places we can go to commune with God, and no unclean thing can enter into the presence of God. So we preserve the sanctity of the house of God by certifying (through local bishops) that people entering in are obeying God's commandments and living in such a way that they will not offend the Holy Spirit in the temple. 
 
2) In the temple we perform religious ceremonies, epic rituals that are deep in their symbolism, substance, and language. The longest of these 'temple ordinances' takes over 2 hours to complete. Everyone is a participant in the temple cermonies. There are no 'observers'.  
 
The purpose of these rituals is to make promises to God to obey the Gospel. So only those who have been prepared and is currently living these commandments should go to the temple to make these promises.  
 
3) Most people who ask this question are actually asking, "Why can't I go see my son/daughter/grandchild/friend get married in the mormon temple?" 
 
The answer to that is because a Mormon marriage is not only a promise to each other, but a ceremony where the couple makes promises to God as well (as described above). If the parties are faithful to each other, they are sealed together for all eternity, not until death.  
 
4) Everyone is welcome in regular Mormon chapels and meetinghouses where we have worship services each Sunday. If you want to learn more about the temple, I would show up there.

"These buildings, different from the thousands of regular Church houses of worship scattered over the earth, are unique in purpose and function from all other religious edifices. It is not the size of these buildings or their architectural beauty that makes them so. It is the work that goes on within their walls. 
 
The designation of certain buildings for special ordinances, as distinguished from regular places of worship, is not new. This was the practice in ancient Israel, where the people worshiped regularly in the synagogues. Their more sacred place was, first, the tabernacle in the wilderness with its Holy of Holies, and then a succession of temples, where special ordinances were performed and where only those who met the required qualifications could participate in these ordinances. 
 
So it is today. Prior to the dedication of a temple, The Church of Jesus Christ of Latter-day Saints invites the public to go through the building and inspect its various facilities. But when it is dedicated it becomes the house of the Lord, vested with a character so sacred that only members of the Church in good standing are permitted to enter. It is not a matter of secrecy. It is a matter of sanctity." 
 
http://www.lds.org/temples/purpose/why/0,11581,1953-1,00.html

Mormons believe that all were members of the Heavenly family before the creation of the universe. Children of Heavenly parents and enjoyed the presence of Heavenly Father and Jesus. When coming to earth they pass through a veil of amnesia for the purpose of building faith. The temples are embodiments of sacred space during their dedications. After 1 year of covenant living as a new LDS Christian all are invited to enter the temple.

Actually if you look at how temples were treated in Biblical times - there were many restrictions about temple usage/attendance that resemble LDS teachings regarding temples today. 
 
Here is a quote from another website (fairmormon.org) that gives a good explanation: 
 
"Why are Mormon Temple ceremonies secret to the public when the Old Testament temple ceremonies were open to public knowledge?" 
 
"Large portions of LDS temple ceremonies are publicly discussed in church publications such as the Ensign, the History of the Church, and the Encyclopedia of Mormonism. There are, however, certain aspects of temple worship that are considered to be of such a sacred character that they are not to be viewed by, nor discussed with, the uninitiated. The same was true with the biblical temple of ancient Israel -- Gentiles were never allowed into the three main temple areas (outer court, holy place, holy of holies) and the entrances throughout the temple complex were guarded by porters and shielded by veils. The vast majority of the Israelites were never allowed to view the ordinances that took place in the temple proper (holy place, holy of holies)." 
 
"Many early Christian groups had ceremonies or services (frequently referred to as the "mysteries") that were only open to those who were faithful members in good standing. Would the critics also condemn them?" 
 
"Jesus also taught his apostles things which they were not permitted to teach to everyone, and this was done in private." 
 
"The Latter-day Saints are merely following a pattern of respect for holy things laid down by Jesus and the early Christians (Matt. 7:6). Latter-day Saints treasure this aspect of Christian life and worship, clearly spelled out in history and scripture."

The temple is holy ground just like the temple of Solomon in the Bible. Only certain people are allowed inside. Just like with Solomons Temple in Jerusalem during the time of Christ.  
 
So only members who are in good standing and are recommended by there local priesthood leader (called a Bishop) are allowed inside to protect the sacred nature of the temple.  
 
You are welcome to tour all meeting houses and the temple grounds! Also when a temple is completed or renovated they have an open house where the public can tour the temple before it is dedicated! This it the only time the public is allowed inside a temple.

Is it true that if someone 'unclean' gets into and touches things in a temple, that those things will be destroyed?

Mary, 
 
I have never heard that before. I do not think that is true.

I thought it sounded a bit excessive.

Temples are very sacred. First of all, not all members can go to the temple. You have to be atleast 12 years old and have a recommend. Other sacred ordinances are performed inside the temple, not just marriage.

So mormons are basically saying that anyone who doesn't believe what they believe are unclean and are therefore not sacred enough. So God doesn't love me enough because I'm not sacred since i therefore can not nor ever will be able to enter the temple. Why would people be not accepting enough to include everyone to try to get everyone to God.

I doubt that anything will have to be destroyed but if unclean things were to enter into the Temple it would be desecrated and need to be closed till it can be rededicated.  
 
Absolutely no one that is not member of the Church with a current Temple Recommend can enter the Temple. In fact Temple's have emergency plans in place in case of medical emergency to have trained temple worker's assist in moving a sick or injured member out of the temple to EMT's who are outside the temple.  
 
I have even heard rumors that in Utah where the membership is in the majority that many fire districts with a temple have a team of fire fighters/emts that are always on call that are endowed temple recommend members in case there is a medical or fire emergency so that they can enter the temple.

Heavenly Father loves everyone, he loves them so much that he protects them from the consequences of entering sacred places unprepared.  
 
We always invite everyone to worship the Lord with us in our meetinghouses and get closer to Heavenly Father and Jesus Christ. The Temple though is not the place for finding Heavenly Father though, it is the place where we receive sacred ordinances that teach us about the plan of salvation that only those who have spiritually prepared for are ready to receive.  
 
To enter the Temple unprepared is a serious transgression and a mockery to Heavenly Father, and Lord will not be mocked. Only persons who have devoted there entire life to living the commandments and have spiritually prepared are ready to enter the temple and do the ordinance work for themselves and there deceased relatives.  
 
You are right you may never step inside the sacred areas of a Holy Temple in this life, I hope that you do have the change through membership in the Church and spiritual preparation to enter the temple to be able to do so, but not everyone will have that chance in this life. Not even every member of the Church will have the opportunity to do so due to spiritual or other concerns.  
 
Not being ready to enter the temple is not a sign of shame or lessen the Father's love for you, all will have the opportunity to receive the blessings of the Temple in this Life or in the Spirit World should they prepare and choose to accept the promise required to obtain them and obey those commandments for eternity.

I'm the only member of the Church in my family and I'm getting sealed to my fiance in an LDS temple in June. 
 
My family doesn't understand why they aren't allowed to go see me get married and I don't know how to properly explain it to them. I've tried a couple times but telling them that temples are holy places, houses of the Lord, and only those who are worthy can enter doesn't seem sufficient enough. 
 
Can anyone help me with a better explanation to tell my family that will help them understand and not offend them?

Marissa - I would suggest that you have a small ring and committment ceremony after the Temple Wedding that your family can attend. You do not have to exchange rings in the Temple but could do it later that day when your family is present.