If 1st cousins can legally marry, why can't we?

Why is it ok for 1st cousins to marry here in Australia and not two people (the same sex) in love and wanting to take their relationship to the next level.

I've heard one comment in particular say "Why can't gay and Lesbians make the same mistake straight people do?"

1. Love is Love...

2. To marry a person of the same sex.

3. The legal right to be at our partners death bed.

4. To adopt a child/our partners children.

5. To have the same human rights as any straight couple in the eyes of the law.