Genesis 1:28 has been used to oppose a great many things, from abortion to contraception to gay marriage. But it takes a fundamentalist (mis)reading of the passage to reach these conclusions.
God, having just created mankind, "blessed them, and God said unto them, Be fruitful, and multiply". The socially conservative reading views this as a command: mankind must multiply, and so abortion or contraception get in the way of this injunction. But rather than being a command, this is instead a description of the conditions God has blessed mankind with, namely that it has the capacity to multiply. In the same passage, God blesses mankind with "dominion over the fish of the sea", which if it were subjected to a similarly fundamentalist reading might lead us to believe trawlermen have special dominion over Heaven.
To effortlessly be fruitful and multiply for all time was the blessed state of mankind prior to the Fall. Moral commands were not necessary in this situation because everything was ordered as God himself had structured it.
Only the one thing God could not foresee or prevent - the Fall itself - complicated the situation. Man's acquisition of free will was the thing that made moral commands necessary, for mankind stands always at risk of annihilation through our own actions, as our generation knows better than any other. But mankind is also capable of the right choices, which will allow us to still be fruitful and multiply, to attain the special happiness available only to the free. Our free will hence comes at the price of work and vigilance; only with these will we be fruitful and multiply. This is the blessing and the burden of the human condition, which the Creation myth captures so well, and of which the fundamentalists are apparently unaware.