Most responsible parents strive to make the lives of their children better rather than worse. Not always successfully of course, but it's the goal, and going to Mars is predictably worse.
> Would the offer of a trip back to earth for any child born on mars go any way to overcoming your concerns?
It might allay the moral dilemma, but it would increase the concerns about the long-term viability of the colony.
>Do we build a wall around Earth and stop them as "illegal aliens"?
This was of course an analogy to the United States, where we put up barriers to prevent emigrants from seeking a better life here.
There's no shame in procreation, but there's shame in trapping people forever in a place where they no longer want to live.
That is an assumption but far from a certainty.
I would not hesitate to do so to any parents that leave a perfectly functional society to raise children as subsistence farmers in an smog-choked, irradiated, arid clime. Opting to raise your children under worse conditions than those one grew up under, and has access to is despicable and selfish.
Would the offer of a trip back to earth for any child born on mars go any way to overcoming your concerns?