They celebrate the freedom to be yourself, love yourself and be with the one you love.
Marriage is for procreation or child-raising.
Imagine you meet someone, you date, have a relationship, think of marriage, raising a family and then be told you can not. It is because the person you love happens to be of the same sex. It's always been law that only a man and a woman can be married. It used to be law that slaves couldn't marry. Heck, slaves used to be legal.