I have a thought experiment for those of you who oppose same-sex marriage. (For the sake of this experiment, and based on the data, I will assume you are heterosexual.)
Suppose for a moment that you wake up and the slate is clean. If you are married, you are now single. If you have children, you are now childless. Everything is different, and you are unaware of the life you lived before.
Like most people, you have a drive to be employed, to find love, to start a family, to raise children, but you are starting anew.
I mentioned above that everything is different. Including your sexuality. Remember the first time you felt attracted to a member of the opposite sex -- that first crush? Imagine that attraction to the opposite sex is completely foreign to you. You do not feel any attraction to the opposite sex, just as you felt no attraction to the same sex when you began to develop crushes on the opposite sex in your school days.
Now, before we move forward, it's important to dismiss the obvious objection to such a thought experiment: our sexuality is defined by a combination of genetic, hormonal, and environmental influences. (When did you make the decision to be straight?)
So, here you are: a clean slate, a desire to move forward in your new life, find love, and start a family. However, despite your completely natural (and almost always unchangeable) attraction to the same sex, you realize that you will never be able to marry, form a family, and enjoy all the benefits afforded to married couples and families.
You see that your legislators are hoping to pass legislation that specifically discriminates against you. Organizations are raising millions of dollars to ensure that you, a taxpaying citizen, do not have the same rights as other taxpaying citizens whose sexual orientations, while different from yours, are determined by the same genetic, hormonal, and environmental influences.
Imagine that you are religious, and you attend church on Sundays. Imagine that your pastor and your congregation do not accept you because of who you are. They denounce you as a sinner, a sexual deviant, and an abomination to God.
Imagine that millions of people honestly believe that your pursuit of the same thing that they have (a spouse and a family) is going to destroy a social institution. They claim that you are harming children (many even claim you are likely to be a pedophile) and that you are a threat to society.
Imagine that you are hospitalized, and the most important person in your life is not able to visit you. Imagine that the person who knows you the best, your soul mate, is not legally allowed to make important medical or financial decisions for you if you are incapacitated.
How does this life feel to you? How is your outlook on life? How do you plan to navigate the rest of your life in a place that will not accept you, a place where you are a 2nd-class citizen, a place where any sort of family you attempt to build will not accepted, a place which has made it clear that you are not welcome?
If you don't feel the least bit of sadness, frustration, or injustice at this point, you're either not being honest with yourself, or you're incapable of empathy.