I'm going to answer a little out of order because I think your second question as the most important, and the answers to the others will logically follow from my answer to it.
Who do you consider marriage important for: - members of your religious group - members of your nation, religious or not (or in a different way) - everyone
One of the main reasons I believe that the institution of marriage is vitally important is because of the children. Children deserve to live in what they feel is a secure environment. Studies have shown that children need the emotional balance of a mother and father. They also need to know that these two are committed to each other and to the family. What better way to demonstrate this than a covenant before God and society that you are husband and wife and you are committed to each other for the rest of your lives.
Another reason is sex. Sex is the most intimate form of affection and should not be trivialized. Sexual promiscuity leads to disease, abortion (murder), and unwanted children. Sex should be the ultimate expression of love and it should only exist between a couple who have vowed to spend the rest of their lives together. When we separate sex from marriage, we only end up trivializing it.
A third reason is that marriage is one of the building blocks that our society, and our country was built upon. "When the United States refused to admit Utah to the Union unless it rejected polygamy in the late 19th century, lawmakers and judges agreed: Marriage was not just a private taste or a values issue or even a religious issue, it was one of the handful of core social institutions that make limited government, and a constitutional republic, possible. Shared family norms enshrined in law were at least as vital to the republic as norms about property rights and democratic government. " (The Weekly Standard)
A final reason I think that marriage is vitally important is because of the stability it gives you in your home life. The husband and wife becoming one is not just a cleaver metaphor, it's actually true. I know that I have this relationship with this wonderful woman who is going to stick by me no matter what. We have made vows before God, our country, and our family that we will do so. Life is chaotic enough, and you need this solid foundation of family. It's hard to describe, but there is a vast difference between the way I was before marriage and the way I am now, and the only explanation is that I have this deep spiritual, emotional, mental, and physical bond with an amazing woman that I'm going to be spending the rest of my life with.
What form of marriage are you wishing to support (and/or require): - marriage by a religious official - marriage by a religious official of a particular church/denomination - marriage as defined by your government - a combination of the above
Marriage is an institution created by God and regulated by the state. So I would say "marriage by a religious official" and "marriage as defined by your government".
If you have views of predestination that include the possibility that some people are destined not to ever 'believe', do you feel that even the '(pre-)lost souls' should follow these rules? (Ignoring the barrier of knowledge: you likely don't, and can't, know who these people are.)
Well, first of all I don't believe that anyone is predestined for hell. Christ came for everyone, he died for everyone, and everyone has the opportunity to accept him as their Savior and be delivered for their sins. However I do believe that marriage, although created by God, extends past the realm of religion into society and is therefore vitally important to all who want to live in society.
How do you see your views affecting people outside of yourself: - these are directives from god, that should be respected by everyone - these are directives from god, that should be respected by his followers - these are societal rules that we should enforce on each other - these are personal rules that simply make sense, and should to everyone
I would have to say all of your possible answers are correct. Marriage is an institution created by God for the good of society, and for most of our history society has accepted this as for the common good and enforced it. It is only in our post-modern world that this, and other long accepted truths, has come under question. Our country is quickly slipping into a quagmire of depravity from which I'm afraid it will never crawl out. I feel that the trivialization of sex and the attack on the institution of marriage are two examples of this.
One final note. I've heard many people use the increasing large divorce rate as evidence in the case against marriage. However, I see divorce, as well as shacking up -- and interest in same sex marriages, but that's another topic -- as effects of the continued trivialization and devaluing of marriage. If we move the institution of marriage back to the place that it should be, people will take it more seriously, the divorce rate will go down, and people will again wait for sex and living together until they are married.