Marriage is between a man and a woman. A Civil Union is a legal union between anyone with the same rights and privledges as marriage.
Wherein lies the difference? Sex.
If you've ever had sex with the hope and intent to make a baby, then you know the difference between Marriage and a Civil Union.
Sex/Civil Union is about love but also pleasure.
Sex/Marriage is about love, pleasure, and the joy of creating new life.
That is the difference and the reason why Marriage is between a man and a woman.