Colonization of the new world is impossible without reproduction of the population of colonists, but the bodies of people, our entire physiology, are adapted for reproduction only under the conditions of the planet Earth. An adult, trained individual of a person can still leave their planet for a short time, but the probability of conceiving and raising a healthy child outside the Earth is assessed as negligible. At least with the current DNA set.
If we take Mars as an example, as the primary target for colonization, then both on the way there and on the surface of the planet itself, the colonists will be exposed to strong radiation exposure. This is tantamount to voluntary and painful sterilization, since the cells of the reproductive organs are among the first to suffer. But this is not so bad.
Microgravity, which is three times lower than Earth's on Mars, will lead not only to muscle atrophy, but also to a gradual loss of bone mass. Even the distribution of fluid in the human body will change dramatically, including the eyes and spinal canals of the colonists - there is no need for blind and helpless disabled people to reproduce. If children do appear, they will surely suffer from immunosuppression, being far from the earthly conditions for which evolution has prepared their bodies.
The way out seems to be relatively simple - it is necessary to make changes in the genome of future colonists, specially to bring out the "Cosmic Man", a new subspecies of Homo Sapiens. However, in addition to the general complexity of the undertaking, there is a big practical problem: we have little idea of how space affects human reproductive function. And if there are already more than enough people willing to voluntarily go to Mars, then it is not yet possible to carry out a set of experiments with bearing a child and childbirth even in near space.