Prop 8 violated Civil Rights. Specifically, the right to enter into a contract, which isn't a basic human right.
Marriage, so far as Government entities are concerned, is just a contract. Under Prop 8 they could still get married, they just couldn't get their marriage contract recognized by their State Government. Which means they would not be granted the legal privileges and responsibilities such a contract affords.
But under US law, denying a person the right to enter into a contract based on their gender (or national origin, race, color, religion, disability or familial status) is a violation of the Civil Rights granted to every citizen. Prop 8 would have violated that by denying people the civil right to enter into a contract based solely on gender. As a CEO, supporting contract discrimination based on gender was directly relevant to his job duties so of course it was an issue.
Free speech is a right. You can't lose your right to claim free speech. Ever. Even hate speech is protected. But, as he was reminded, the right to speak freely only protects people from consequences imposed by Government. Employers, customers, investors and the general public are not the Government and the first amendment doesn't guarantee any protection from them.