Basically it means to fuck off. Use this phrase as a euphemism when you don't want to be obscene in telling someone that their presence is not desirable.
When the boss of Taco Bell went to his workers and asked them to take a pay cut, they all told him to go forth and multiply.
The only passage in The Good Book that hormone-steeped young studs find of any pleasure.
A lot of Bible verses can be intentionally misinterpreted and then shamelessly used to further a selfish/evil cause or desire; "go ye forth and multiply" is a prime example, since it seemingly justifies a teenage boy's pleas to a chick --- even a pious Christian girl --- that she spread her legs for him, since "God told us to procreate, so you would be disobeying His Word if you refuse me".