Because of some ill-devised MONSANTO-chemicals, human males in the 21st century were able to become pregnant as well. Which took place automatically. And, just like domesticated mice, the following incest-bred increased their reproduction-rate by +300%... as you can see that led to a real heavy motive to become a space-faring civilization in order to find some places to distribute the people around...
Then we met other alien races, and although down by law, there were some multi-racial mass cuddlings lalala the results of I don't want to speak of....
Just use your imagination^^