Sheldon kissed Amy on her lips in "The Big Bang Theory" Season 7 Episode 14, "The Locomotive Manipulation." It is a kiss that no one saw coming, including Amy. Before the kiss, Amy was complaining about Sheldon being rude to her during the entire evening, which was set for a Valentine dinner, because he hardly spoke to her.
In the episode, Sheldon befriended a man, Eric, who shared his love for trains. This new friendship came at the cost of Amy's romantic dinner plan. The video clip of the kissing scene shows Amy asking Eric to leave as she wants to have a talk with her boyfriend. Sheldon can be seen objecting to her rude tone. Amy tells Sheldon that she "deserves romance."
The demand for "romance" triggers a reaction, as can be seen in the video clip. Sheldon says: "... well if you want romance, lets have romance." His rude romantic gestures begin with taking a sip from a glass of wine and gazing into Amy's eyes. Then, Sheldon kisses Amy, saying "kissing is romantic."
At first, Sheldon can be seen kissing Amy, standing in an awkward position. There is a distance between the two. Soon, he stands straight and comes close to Amy, without breaking the lip-lock. After the kiss, both Sheldon and Amy wear an affected look. "That was nice," Amy tells Sheldon. "Good" is Sheldon's reply. The kiss turned an annoyed Sheldon into a well-behaved boy-friend.
Later in the episode, Sheldon said to Leonard that Amy's lips tasted like a brownie. However for Sheldon, kissing Amy is no different from making a new friend and listening to banjo played by a train conductor who has four fingers.
Will this kiss change Sheldon and Amy's unique relationship? It might add a new facet to their relationship, but it looks highly unlikely that the genetic of Sheldon and Amy's will completely change.