Adam and Eve were commanded to multiply and replenish the earth.
28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
29 And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat.
30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.
The same commandment was also given later to Noah and his sons.
1 And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth.
2 And the fear of you and the dread of you shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered.
3 Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.
In both cases the commandment to multiply and replenish the earth is followed by a commandment to have dominion over the earth (perhaps the two are related, but that's for another discussion). I point that out because surely the commandment to have dominion over the earth is still applicable to us. Does that mean the commandment to multiply and replenish the earth is still applicable? Its pretty significant if having a family is a commandment as opposed to being a suggestion or just optional.