Currently you can stream The Case Against 8 with an HBO subscription. We've included a link to the HBO Max page on Amazon Prime Video where you can join HBO Max free for 7 days if you'd like to stream the documentary.
If you don't have HBO Max and you don't want to use that service then you can buy or rent The Case Against 8 to watch online from platforms such as VUDU, Google Play Movies and YouTube for only $2.99 to rent the documentary.