Originally intending to bless the creation of His own likeness, God assigns them to a creative labor to mirror His own (cf. Gen 1:26; 2:15). God's work thence became the very frame for man's work: the creature mirrors the Creator, working as he ought, with the hope of like Sabbath rest to God's (to be won when the Creator judges his work "good" - like His own). As originally conceived, this blessing/command envisioned a sinless creature who, once confirmed in righteousness (through mirroring God via His Creation-covenant of works), would populate the earth with a holy people, a city of righteousness that would hold faultless dominion over the world (as per the Gen 1:28 command).
Alas. Such was not to be the case.
Yet by common grace, the blessing/command to be fruitful and multiply was extended to fallen mankind who now being corrupt, quickly learnt to use the blessing to their own ends - to make names for themselves and increase their own houses (cf. Gen 4:17). Instead of glorifying the Creator, man, the creature, deigned to usurp His place through self-deification.
But, with the advent of Christ, the command is renewed and is finally able to find its true and lasting fulfillment. As the Last Adam, the first born of the New Creation, Christ fulfills the Genesis 1:28 mandate in His dominion (cf. Rom 8:34 and Eph 1:20). And so, in His resurrection, Christ opens up a New Creation and reiterates the blessing/command in Mat 28:18 with the Great Commission. In the New Creation, the "fruit" takes on spiritual meaning, finding spiritual fulfillment rather than carnal. Now, it is the spiritual children who are counted as fruit (over and against those who are solely children according to the flesh).
So, while the Genesis 1:28 mandate to "be fruitful and multiply" is strong in command today, it finds renewed and redeemed expression in the invitation to "make disciples of all nations." Thus, it is by the sharing of the Gospel that we, as post-Ascension Believers, endeavor to fulfill the blessing/command.