The rabbinic commentators make a distinction between the first instance of these words, which they take as a blessing, and the second, which they understand as a command (the Even HaEzer volume of the Shulchan Aruch begins with this command).
The nature of the command is linked with the previous verse which forbids murder. Rashi follows the Midrash in telling us that someone who refuses to procreate is like a murderer. Such a person has wilfully prevented a human life from coming into being.
The problem is not with those who endeavour to have children but do not succeed, but with those who say, as a young person told me years ago when we were having a discussion of serious world issues in a London youth club, “The world is in a terrible state. It would be a crime to bring children into it!”
The Biblical answer, which I tried to explain to the group that evening, is quite different and much more positive: “If the world is in a state, we can all help to make it better by giving birth to children and helping them to make their contribution towards improving it!”
There is a Yiddish saying, “Every child brings its own blessing into the world”. You can argue if you wish that the world is imperfect; the Yiddish saying says, “But think of how much worse it would be had so many minds, hearts, talents and lives been prevented from coming to birth”.
If the world is not yet Utopia, its only chance is to nurture new generations that will bring new blessings with them…