From a comments thread on a website (concerns same-sex marriage and what 'traditional marriage' really is. The site contended that 'traditional marriage' isn't a good thing anyway. I have tried to define what marriage is in this context and why it ought not be derided)
Marriage exists for the same reason sexuality exists. The two are linked, and anyone who knows how we define the Law of Chastity would admit this.
Despite sexuality in marriage as a means to enhance the emotional bonds within that marriage, the real purpose of sexuality and ultimately the emotional bonds involved, is the proper and loving upbringing of children.
Being a product of a divorced-parent upbringing, I am absolutely aware of the emotional difficulties such children will face growing up. When you want to maintain and project the divine love and trust that the gospel is meant to instill, it is such an enormous help to have learned that love and trust in a stable family with the possibly imperfect, yet forever committed relationship of service and love between parents as the chief example.
If you believe in God, then you believe that he had some hand in our creation. If you believe that, then the fact that such a divine thing as the creation of new human life requires the biological union between man and woman is clear and incontrovertable evidence of God's intentions for family relationships.
If you don't see the importance of children, can't grasp the central role families play in the plan of the creator, the Father of life, then I can't understand what place the gospel has in your life.
I sympathize with those who are concerned about the history of mistreatment of women. I could also mention the centuries and centuries of use of false religion to maim and abuse - women and men. We live in a blessed age, relatively speaking, it is wonderful that society sticks up for the abused more and more. However, that a woman should take a certain role in a family, that she might not always like it, and might not always appreciate her husband, and might be frustrated at what she has given up in life, that the one thing that is meant to be her blessing - her children - might often seem a burden and annoying, that any number of difficulties in life might discourage her - none of this justifies the argument that true happiness and life's purpose are meant to be found elsewhere.
There are countless men who face challenges perhaps just as daunting in life (as annoyed as you are with him, imagine how that makes him feel about you). I'm not trying to make an equivalence, I'm only saying that, of course, life is hard.
Ultimately, marriage needs to be understood as a sacrifice. It is a sacrifice, and for latter-day saints, it is one that we are constrained to take upon ourselves because of the knowledge we have concerning its blessings.
The hardest part about marriage is that it involves someone else, but the gospel is all about other people.
So to recap. Marriage and sex, inseparable, are inseparably connect to the (eventual) creation and maintaining of families and children. That's where people come from, and that's where society comes from. It's also where society gains the emotional and moral stability that engenders civilized treatment of others. Eventually, if we're all victims, we won't care so much any more about each other.
No-fault divorce is stupid, and in society it might be tolerable, but considering the divine role of marriage in an eternal plan involving eternal families in eternal relationships, such a lack of commitment is troubling.
That's why, despite the good reasons for sustaining clear counsel from a prophet, I think gay marriage shouldn't be allowed. To extend certain rights to two individuals who want to share a life together, despite their genders, is one thing. To redefine marriage so that it no longer applies to the divine institution for family creation is something else entirely.
When sexuality (and marriage) become something besides what the divine intention was, society will fall apart spiritually (not to mention emotionally/psychologically as fewer and fewer new children are born in loving/stable environments). If individuals are compelled to use/express their sexuality in a different way, that's not the business of the state. However, defining a clear place for marriage and families in society is.