The 26-year-old 'Jersey Shore' star wrote: "Bristol should keep her uneducated, ignorant mouth shut. If you're living in the past you wouldn't have a kid without marriage. Hypocrite. It's 2012!
"My best friend will get married one day and not just have civil union with his man. Stop hating people."
During his interview with ABC News' Robin Roberts, Obama explained how his daughters inspired him to think more seriously about the topic.
He said: "Malia and Sasha, they have friends whose parents are same-sex couples. There have been times where Michelle and I have been sitting around the dinner table and we're talking about their friends and their parents and Malia and Sasha, it wouldn't dawn on them that somehow their friends' parents would be treated differently.
"It doesn't make sense to them, and frankly, that's the kind of thing that prompts a change in perspective."
Bristol argued: "Sometimes dads should lead their family in the right ways of thinking. In this case, it would've been nice if the President would've been an actual leader and helped shape their thoughts instead of merely reflecting what many teenagers think after one too many episodes of 'Glee'."