A pro athlete who waits until his career is over to declare he’s gay, the way John Amaechi just did in a new book, is yesterday’s news.

He has something to sell, little to lose and does less to advance the cause he’s fighting for – helping sports get over its homophobia – than he might have. There’s no dishonor in that. It’s what happens when you work within the system: You spark debate, maybe change some minds over the long haul and pick up a few bucks for baring your pain.

So no disrespect to John Amaechi.

But it’s been done before.

Homosexuality has been one of sports’ little morality plays since the ancient Greeks made contests of running, jumping and boxing, then gossiped about them afterward. Long and loud as the discussion has been at times, nobody from any of the big team sports has gone on the record while still playing the game. Not much will change until he does.

“It’s a shame all gay people can’t turn green for just one day, so everybody’s jaw would drop, and then maybe we could move past a lot of this stuff,” said Jim Buzinski, co-founder of Outsports.com. “Until then, we’ll take another small step forward. It’s always a positive, and I applaud John for what he did. In terms of encouraging others, everybody knows their own circumstances and deals with it. …

“I think the fears are overblown,” he added, “but until the first person comes out, we won’t really know.”

No.

Even so, it’s getting easier to make an educated guess. It’s difficult to imagine too many owners balking at the idea of an openly gay athlete on their team, unlike Jackie Robinson breaking baseball’s color barrier, back in the day. Most, in fact, would line up with their wallets out for the opportunity to market him.

Though his players weren’t uniformly enthusiastic, commissioner David Stern said about Amaechi’s book: “We have a very diverse league. The question at the NBA is always ‘Have you got game?’ That’s it, end of inquiry.”

A call to NFL headquarters produced a similar response. “Society is more open and tolerant than it’s ever been. I don’t think anybody would care,” spokesman Greg Aiello said, “other than how he was performing.”

Last June, the NFL flew in former defensive lineman Esera Tuaolo – who like Amaechi came out after retiring – to speak to the incoming rookie class about sexual orientation. That was after Tuaolo talked to the league staff about the same topic in New York a few months earlier.

Considering how little the NFL likes being surprised, Aiello was asked whether he could imagine a football player coming out any time soon. “I’m sure it will happen someday,” he said. “I don’t see why not.”

You could have come up with a thousand reasons why as recently as four years ago, when an online and print rumor campaign prompted Mike Piazza, who was playing for the Mets at the time, to deny he was gay.

Almost everybody still was fleeing the stigma attached, convinced it could spell the end of a career.

Thousands of ballplayers have worked, traveled, showered and dressed in the same locker room together for more than a century. Most understand now it’s a numbers game, although something then-Phillies manager Larry Bowa said about numbers during the Piazza episode still applies.

“All I know,” Bowa said, “is that if he hits .340, it probably would be easier than if he hits .220.”