As the OP points out, the verse is stated as an imperative. It should be understood as both a blessing and a commandment - a mitzvah. The blessing (certainly more than mere permission) is given to the unnamed man and woman in Gen. 1, so is often taken to apply to humankind generally.
However, it is important to keep in mind that this blessing/commandment was given prior to the first sin (also called the Fall). In the OTA Jews generally understand Gen. 1:28 as applying to everyone and thus Jewish men were taught it was their duty to marry. Paul, however, understood humanity as essentially fallen. For him, even the children of Christians are not born under God's blessing but require a savoir. Jesus, the new Adam, did not marry but went to the Cross to save those who believe in him. Thus Paul taught that marriage was a secondary choice for those awaiting the Second Coming.
1 Corinthians
8 To the unmarried and the widows I say that it is good for them to
remain single, as I am. 9 But if they cannot exercise self-control,
they should marry. For it is better to marry than to burn with
passion.
Conclusion: Religious Jews see marriage as both a blessing and a duty commanded by God. However, following Paul's teachings, Christians often interpret Gen. 1:28 as being given specifically to Adam and Eve as the parents of humankind. Because of the urgent need for procreation, it was a commandment to them as well as a blessing. But for Christians it is not a commandment. Those who await the return of Christ need to decide for themselves whether to follow Paul's preferred course and remain single, or to marry.