Most Helpful Girl
Besides fighting for basic human rights, which should be a given; here's a link to help enlighten you on the extras marriage entails
Also in regards to your comment about children- yes it's true that it's difficult for homosexual couples to adopt, however marriage would be a step closer in the direction needed to gain those rights as well. Change isn't instantaneous. You need progression to reach the ideal result.
In case you have any concerns on regards to homosexuals raising children
And an example of why the marriage title is very important, and why not being able to marry is discrimination.
I hope some of this helps you see why it's such a big deal.