It should first be noted that the grammatical construct in the Hebrew of "be fruitful" is that of a plural, masculine imperative (i.e command) - not a second person indicative (merely blessing). That basically kills the argument that this is meant merely as a blessing. Their argument seems odd anyway, since I've never heard the same people argue that "subdue the earth and rule over it" was ONLY a blessing. Yet these commands were given in the same breath.
So, what is the nature of this divine imperative statement? Is the command still valid today?
An interesting take on the passage is given in the Formula SD VII:75-76, citing Chrysostom, showing that it is the Word of God that is effective in the Sacrament.
For where His institution is observed and His words are spoken over the bread and cup [wine], and the consecrated bread and cup [wine] are distributed, Christ Himself, through the spoken words, is still efficacious by virtue of the first institution, through His word, which He wishes to be there repeated. 76] As Chrysostom says (in Serm. de Pass.) in his Sermon concerning the Passion:Christ Himself prepared this table and blesses it; for no man makes the bread and wine set before us the body and blood of Christ, but Christ Himself who was crucified for us. The words are spoken by the mouth of the priest, but by God's power and grace, by the word, where He speaks: "This is My body," the elements presented are consecrated in the Supper. And just as the declaration, Gen. 1, 28: "Be fruitful, and multiply, and replenish the earth," was spoken only once, but is ever efficacious in nature, so that it is fruitful and multiplies, so also this declaration ["This is My body; this is My blood"] was spoken once, but even to this day and to His advent it is efficacious, and works so that in the Supper of the Church His true body and blood are present.
Here God's imperative statement is seen as more than a command - not less than one. In other words, it is the power behind the command which is fulfilled as well when God speaks.
Consider this from the Roman Confutation pt. II, art. II, with regard to the marriage of priests:
At that time a command concerning the procreation of offspring was given to fill the earth, but now that that has been filled so that there is population pressure, the command no longer pertains to those able to be continent.
To this the response in the Apology [XXIII.8] is:
Our opponents trivialize these arguments. They say that in the beginning there was a command to fill the earth, but now that the earth has been filled marriage is not commanded. Look at their clever argument! The Word of God formed human nature in such a way that it may be fruitful not only at the beginning of creation but as long as this physical nature of ours exists. Likewise, the earth became fruitful by this Word [Gen. 1:11]: ‘Let the earth put forth vegetation: plants tielding seed.’ Because of this ordinance, the earth commenced to produce plants, not only in the beginning, but yearly the fields are clothed as long as this natural order exists. Therefore, just as the nature of the earth cannot be changed by human laws, so neither can human nature be changed by vows or by human law without a special act of God.
And here are the words of Luther on this verse of Scripture, from which I believe Melanchthon likely fashioned his wording in the Apology.
For this word which God speaks, 'Be fruitful and multiply,' is not a command. It is more than a command, namely, a divine ordinance which it is not our prerogative to hinder or ignore. Rather, it is just as necessary as the fact that I am a man , and more necessary than sleeping and waking, eating and drinking, and emptying the bowels and bladder. It is a nature and disposition just as innate as the organs involved in it. Therefore, just as God does not command anyone to be a man or a woman but created them the way they have to be, so he does not command them to multiply but creates them so that they have to multiply. And wherever men try to resist this, it remains irresistible nonetheless and goes its way through fornication, adultery, and secret sins, for this is a matter of nature and not of choice.
In the third place, from this ordinance of creation God has himself exempted three categories of men, saying in Matthew 19:12, 'There are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven.' Apart from these three groups, let no man presume to be without a spouse. And whoever does not fall within one of these three categories should not consider anything except the estate of marriage. Otherwise it simply impossible for you to remain righteous. For the Word of God which created you and said, 'Be fruitful and multiply,' abides and rules within you; you can by no means ignore it, or you will be bound to commit heinous sins without end.
[Luther's works, vol. 45, The Christian in Society II, The Estate of Marriage, pp. 15-18]
"Be fruitful and multiply" is both command and blessing - more than a command - and still applies to man "as long as this physical nature of ours exists.".