Another life experience that has forced me to think about this is my experience in Church leadership. I served as a branch president of a student branch at the University of Illinois for over four years, and I served as the bishop of a family ward in a university town for almost six and a half years. I have performed a number of weddings myself, and I have counseled people before and after their making this critical decision about what kind of wedding they would have. I also served over four years in a stake presidency, which presented me with additional opportunities to counsel with Church leaders and inviduals who were struggling with these things.
I certainly don't have all the answers, and I recognize that each couple has unique circumstances that need to be considered. But I hope that my thoughts on this page will be of benefit to someone and that my own experiences may add something to the discussion.
Worthiness is only one of several reasons why someone might not be able to enter the temple, and in the case of non-members, it is NOT the reason. So not only is it inaccurate, it has the added drawback of being insulting. Giving this reason is essentially saying to someone that he or she is a worse person or a more defective person than the Latter-day Saints who are permitted to attend the wedding.
If you stop to think about it, the non-member is not being denied access to the temple because someone has met with him or her and discovered moral issues that are severe enough to disqualify him or her from attending. This is also true for LDS teens and children. They are not being denied access because of worthiness either. Nor are adult members of the Church who are recent converts. In all of these cases, people are being denied access to temple weddings for something other than worthiness.
So what are the reasons that someone is denied access to the temple? It seems to me that there are at least three: covenant status, preparation, and worthiness.
Latter-day Saints believe that a temple is the House of the Lord and that the temple is intended as a place of worship only for those who have entered into a special covenant relationship with God. This is not simply a matter of belief in God, nor is it a matter of sincerity or intent. Latter-day Saints believe strongly in the idea of formal covenants between man and God, covenants that involve specific commitments on the part of the individual and that are formally administered and recorded. This is not a matter of moral goodness, though it is hoped and expected that anyone entering into such a covenant agrees to live at least a minimum standard of moral worthiness. Latter-day Saints do not believe that those who have not entered into this covenant are necessarily any less moral, less spiritual, or less believing. But Latter-day Saints believe that God has once again created on the earth a specific community of believers who are formally bound to each other and to God by specific commitments and covenants that have been administered by those to whom God has explicitly given His authority for such a purpose.
The primary reason that non-members cannot enter temples is because they have not yet made that formal covenant with God and have not yet bound themselves into that community for whom the temples were built. They are not being denied access because they are bad people or because they have done anything specifically wrong. They are not being denied access because they are not as moral or as good as a Latter-day Saint who IS being allowed to enter. They are not being denied because they don't believe in God or because they don't have faith or because they lack some spiritual gift or attribute. They are not allowed to enter because the temples were built explicitly for the special worship of the community of saints who are bound together under the formal covenant given to the community by God Himself, and those people have not yet joined that community.
A second reason people are denied access to the temple is that Latter-day Saints believe God has required that people reach a certain level of maturity within the Gospel community before entering into the temple. For Latter-day Saints, the temple is not merely a building for offering prayers of gratitude, petition, and praise, though it serves that purpose as well. In addition to those forms of worship, the temple is a place for members of God's community to enter into additional covenants with God. These additional covenants involve commitments by the individual to live the very highest moral laws as well as a commitment to entirely and forever submit to Jesus Christ as Lord and to sacrifice all for His sake.
These covenants are viewed as extremely sacred and binding by Latter-day Saints. They are seen as covenants that should only be entered into by those who understand their seriousness and sacredness. These covenants are intended for those who have demonstrated that they are willing to live the more general covenants associated with baptism and who have an understanding of God's plan for His community of Saints.
It is primarily for these reasons that children, youth, and new coverts to the Church are not allowed full access to the temple. Again, it is not a matter of morals or of being good person or of personal spirituality. It is a matter of the person being fully prepared and willing to make the covenants associated with temple worship.
The third reason people are denied access to the temple is lack of worthiness. It should be noted that worthiness simply isn't an issue for non-members, new converts, and youth. Worthiness only becomes an issue for a person who is already under covenant with God and who has the apparent understanding and willingness required by God for those who may enter His temple. Adults who have been a member of the Church long enough to understand and appreciate the temple covenants must be living a life that meets the minimum requirements set by the Lord for entering His temple. The requirements include both elements of moral goodness (such as living an honest and chaste life) and elements of willing self-sacrifice (such as paying tithing, attending church, and serving within the community of God). The individual who enters the temple must also believe in God and Christ and believe that the temple and its covenants are indeed elements of worship given by Jesus Christ to His people.
So the "worthiness" associated with the temple is really not just a matter of how well-behaved a person is, but it is in reality an issue of a moral life, a demonstrated commitment to Jesus and the community of saints, and a faith in God and His work on the earth.