It may already be moot. There’s a chance that some of the probes which have been sent to Mars weren’t properly decontaminated before they left Earth, so there may already be Earthly microbes running about.
Yes, Ken Mattingly (Gary Sinise in the movie) was exposed to German measles and was pulled off the crew three days before the launch. He didn’t get sick, but NASA certainly didn’t want to take the chance.
Deke Slayton was pulled from the active astronaut corps for years despite being selected for Mercury because of a heart murmur. He eventually flew on the Apollo-Soyuz flight.
So NASA has certainly tended to err on the side of caution, especially in the early days. I think the very long mission to Mars and back would obviously entail a “best of the best” selection process similar to the Mercury astronauts.
I recall Gene Krantz referring to a flight surgeon who wanted to scrub if Deke’s heart beat varied; “I fired that sin of a bitch.” 
What does the ABA say about this? How would a case of sex discrimination case be litigated? And racial discrimination on Mars-how to litigate?
Conflicts between green and red Martians are traditionally settled at swordpoint.
“Screw it!” exclaimed John Carter as he threw aside his broadsword, drew his radium pistol and blew his far larger opponent through the wall.
I have a feeling NASA would get the government to immunize itself against discrimination claims during off-world operations.
We actually have a good model for all of this: Nuclear Submarines. They face the same combination of hostile environment, limited resources, and isolation. While a sub can more easily go back to port to deliver an ailing sailor, I’m sure they have provisions for wartime when they can’t. I’m sure the Navy has already thought through the implications of many scenarios (and more besides, the Mars mission won’t be carrying WMDs), including any legal ramifications. NASA would do well to observe and copy them.