Okay, I'll take your challenge.
Social Definition should be: A Union between a Man and a Woman.
Justification:
Marriage between a man and a woman is ordained of God. Marriage between man and woman is essential to His Eternal Plan. (Source: http://www.lds.org/library/display/0,4945,161-1-11-1,00.html)
Now, that is my view, based on careful study and thoughtful inquiry and seeking after truth. I recognize that others may not understand this the same way. I value their individual perspectives and respect and love them still the same.
I also recognize that there is a difference between Eternal Marriage covenants and earthly contracts which have no sway in the eternities.
Therefore when it comes to the discussion of legal matters, we must look deeper into what benefits legal recognition of marriage has to society. I put forth that civil marriage is based on the importance of gender as an aspect of ones identity and personal attributes and that a union between a man and a woman allows for great synergy and creates the appropriate corner stone for families- and the raising of children. The synergy between male and female is essential to the benefit of marriage to society and the appropriate rearing of children.
I love and respect those who are attracted to the same sex, and I support their legal right to choose their own lifestyle. However, I cannot support a change in the legal structure of the family to cater to their individual tastes and desires, nor can I personally condone action that is inconsistent with the commandments of the Almighty God of Israel.