Season 6, Episode 23: The Love Spell Potential.
"What’s changed? In some ways, everything you know about Sheldon you begin to look at differently if he’s actually willing to consider sometime in the near future having a physical relationship with Amy. He’s willing to step over a line that he wouldn’t cross before and engage in this imaginary level of it. He’s decided that this person is important enough to him to try to make a change.”
— Jim Parsons.