They're related. First is that we don't know if humans can survive on mars. Second is that even if they can it would require an unsustainable level of support from earth.
We certainly do know that humans could survive on mars and it wouldn't require them to become mole people lacking a decent life either. We can't be sure what the entire biological effect of 38% g will be but we could absolutely survive and adapt to whatever that is. Zero g is not the same thing. But this isn't going to go away until we do it. Radiation is another one of those fears that won't go away until we deal with it. This is why we need frontiers... to deal with these pansy fears.
Then you have those that think without earth support the martians would just wither away and die. That's just silly. Not totally unsurprising when you hear of plans to send a half dozen or less. Yes, it is possible to guarantee failure by not making a good first commitment. The fact is, because of the challenges they would face, if we give them a decent start they would probably pass us up in technology in a hundred years or so. The university of mars is going to have to provide a real science and engineering education. Economics too. 'Gender feelings are hurt' studies will probably not be offered. Victimhood 101 is definitely out the window. The human race really needs to be shook up. We live in a sea of false assumptions. Killing these two would just be a good start.
I realize I'm making an assertion I can't prove. We will have to go before it can be settled.
I have an assumption that Zubrin and Musk both claim is false. I can see cost to LEO per person going as low as $2m. I can see the cost per person to go to mars orbit going to $38m. I can see the cost per person to land on mars being $22m. I add those up and I get $62m per person. I can even see how that could go lower in time. I just don't see, regardless of my fantastic imagination, the price per person for a ticket to mars going to $500k. I just don't know how you get there? I can see perhaps $30m. I have a hard time getting any lower.
I've outlined how it's economically possible to settle mars even at $100m per person. Get anywhere near $500k and I would expect an explosion of folks heading out into the solar system. Update: Newer argument.
Perhaps Zubrin and Musk have nuclear ambitions the rest of us don't know about? Just kidding.
I strongly suspect if we are going to colonize mars some company is going to have to accept a few billion dollar cost with an uncertain return. I believe that people with both the vision and resources are already working on it. I hope to see it in my lifetime. Government is never going to do it. Spending other peoples money is not visionary. It takes liberty to make it happen.
Sometimes I just need to rant.
Then there's a third false assumption that earth must have some mars export or mars becomes economically non-viable. This is just economic ignorance. It's exactly the same as saying the earth is non-viable economically without some trading partner. But you can't argue this one with some people.