Don't take this in any way as a disparagement of Jackie Robinson. Without question the man is a hero. But he's a hero because he chose to put himself in the spotlight. You say gays can just hide within society, but Jackie Robinson could have hidden himself as well to a large degree. No, he couldn't change the color of his skin, but he could have done what so many other black men felt forced to do and walk around with his head down, not make eye contact, respond "yes sir" when called "boy, obeyed the Jim Crow laws, etc.
Robinson is a hero because he rejected the place American society wanted to keep him. He's a hero because he subjected himself to scorn and ridicule. When he was offered the spot on the Dodgers he took it knowing full well that he would hear vile things shouted at him, that things would be thrown at him, that he'd be harassed and hounded.
Is Robinson unique? No. For most of this country's history, white Protestant males have held power. Not wanting to lose power, they set up barriers for the rest of society. Find some old newspapers and read what white Protestant males in power in Mississippi said about Hiram Revels, read what Southern white Protestant males said about Frederick Douglass, William Lloyd Garrison, Harriet Beecher Stowe, read what white Protestant males in DC were saying about women like Elizabeth Cady Stanton, Lucretia Mott, Susan B. Anthony, and Alice Paul. All of these people fought to break through barriers and some subjected themselves to atrocities much worse than Jackie Robinson had to endure. All heroes. All putting their lives on the line to open doors that had been previously closed. And all made this a better country for those who came after.
You said that gay players in the NFL today "are not experiencing what Jackie Robinson experienced." This is true. It is true because no openly gay man is playing in the NFL. None have had the courage to put so much on the line. And that's why what Michael Sam is doing is heroic. This could all go very badly for him but despite the risks, he's putting himself out there openly. If he's successful and if he is accepted, his action will open the door for more openly gay men to play football, just as Jackie's action opened the door for more black men to play in the major leagues.
Remember that back in 1947, to most of America, Jackie Robinson was a troublemaker who didn't know his place.