I agree there should be a moratorium on extra-Earth births for awhile. But eventually, it is reasonable to expect that we can do it safely. We will continue to learn how to modify people at a genetic level, to adapt them better to off-Earth environments.

Also, no one has experienced "low" gravity for an extended period, so we don't know its effects. People have experienced micro/zero gravity - in orbit - for extended periods, but not lunar or Mars gravity for more than a few days. It could be that a little gravity, combined with exercise, is enough to prevent atrophy. We don't know. But again, by the time this becomes reality, I expect we will have medical interventions. E.g., there are drugs in trials that simulate exercise: https://www.technologyreview.com/2008/07/31/219451/new-drugs-mimic-exercise/

and we are only in the early days of generic manipulation.