We want to preserve Biblical standards, that specifies marriage as being between a man and a woman.
Why is this important?
Whether you call it religious, spiritual, God fearing, God loving, believers, or however else it's best identified for the individual; this community deserves to preserve Biblical standards.
It is possible for the GLBT community to have equal legal rights from being in a union with one another, for tax purposes, work benefits, etc. without redefining marriage for all believers everywhere.
God's Word does specifically state that marriage is between a man and a woman and was originally ordained for the purpose of reproduction. Let's respect and protect this specific union.