My guess is the NFL breaks it first. Followed by baseball and the NHL. But I hate the NBA so my biases may be at play here.
I've always thought baseball would be first.
Maybe, but football players seem to be a little more open minded than baseball players (yes, that is a low bar)....something to do with them spending time at college whereas baseball players don't tend to go to college and spend more time in a more closed, ....ahem..."closeted" environment. Granted, most top football athletes in college live a fairly segregated, non-typical college life, but still, they are much more likely to run into "out" people than the other 3 major American pro sports athletes.
I reckon it won't happen anywhere anytime soon, but I hope I'm wrong.