"The Mote In God's Eye" - synopsis and spoilers
 

First contact story. Aliens are discovered. We refer to the aliens at
Moties. Humans visit the planet of the Moties. We bring a few Moties back to earth. But we discover that the aliens have to fuck all the time. If the Moties don't get pregnant they die. So every so often a huge war takes place to control the over population.

We blockade their planet so the Moties don't come here to overpopulate earth and start
warring.