Who was a more liberal president, Carter or Obama?

[Oswald Acted Alone, You Kook]

It depends on the context.

Do you mean relative to their era, or in comparison to one another?

Obama probably "wins" in both categories. Carter was challenged by two prominent primary opponents (The best-known Senator in the country, and the Governor of the largest state with a Democratic executive.) He failed to endorse gay marriage at any point in his presidency.

That's because it was the 70's. I'd bet less than 10% of the US Population supported SSM before 2000.