Celebrities have freedom of speech just like anyone else. As long as they accept the possible consequences of their actions, both for themselves and society, they should be able to express whatever viewpoints they want. From a societal standpoint, though, people should probably think twice before doing something just because some famous person says it's right/wrong. People need to make up their own minds, preferably with evidence to support their conclusions.
If a celebrity wants to take advantage of the platform they have to speak out on an issue, sure. I don't think we should think that they have an obligation to, so if a celebrity would rather not say anything, that's fair. If they want to use their status to talk about an issue, they have as much right as any other person should. I also think that a morally aware person would use any resource they have to correct a perceived wrong, especially one that is on such a large societal scale.
Yes movie and sports celebrities should be among the first Americans to step-up and express their opinion about marriage equality, no matter which side of the issue they are on. By expressing their views they lend importance to the issue. In expressing their opinions they force the rest of America to take a long hard look at the question of marriage equality.
A celebrity is always looked up to. People like what they have to say, and they seem to believe they know all. They can use their status to make a good argument about marriage equality, and it will stick. However, if you really think about it, so many star marriages end in divorce, so they probably arent the best model.
Hollywood is full of cultural Marxists anyway. They're going to speak out on gay marriage and "equality" anyway, but I don't believe they should. Using popularity as a tactic to promote certain things will inevitably halt the progress of this nation. Imagine if a celebrity whom everyone loved was advocating for the end of the Keystone XL pipeline, the project would be decimated and we'd have to continue sucking up to the Middle East for our foreign oil.
I personally don't give a .... For celebrity opinion. An actress in a movie has no business giving me moral advice. That would be like asking my plumber how to fix my car, she/he would give me a very weird look and ask me what I was smoking (unless he happened to be a mechanic, which is possible).