They surveyed the landscape from above. It was prime property for development. There was only one problem. It was occupied already and the current tenants would have to be removed. Of course, there was the heavy-handed approach to remove by force. But they realized that would destroy much of the pristine paradise they saw below them. No, they decided the more gentle, albeit more slow, approach was preferable. They calculated, entered into the water supply, that the parasites created to make the sex act unpalatable, would cleanse the third planet from the star of tenants in less than 50 years.