I'll answer why people should care.
The US is becoming very liberal. Very "if it doesn't hurt you, why do you care?" Should I care if you smoke pot? No? Should I care if you snort coke? No? Should I care if you shoot heroin? No? After all, you're just hurting yourself, right?
Well, maybe I should care just a little. First, you're going to become an unproductive citizen. Second, you're going to have health problems. Now, with Obamacare, I get to pay for your healthcare.
That said, I don't care who gets married. But I should because I believe "If it's not hurting you" isn't a good enough reason to not care.