It's a status/benefits issue. Assuming one lives in an area where civil unions posses the same legal status of traditional marriage (many don't; I lived in a state where it was the same, now gay marriage is legal), then the reason for getting married is to appear the same as traditional couples. If traditional couples suddenly opted for civil unions, you would see the gay marriage debate wither away.
But if we walk in the light, as He is in the light, we have fellowship with one another, and the blood of Jesus, His Son, purifies us from all sin.
