I have read the books and seen the series. Here's my take:
1) In the book, Clegane is IN HER BED DRUNK, CRYING AND COVERED IN BLOOD. Not a great scene for a teen to walk into. And he didn't expect her to be there. She's supposed to be hiding in Maegor's Holdfast. He went there to experience what HE thinks is his last time to "be" with her.
He also grabs her quite violently, throws her on the bed, and holds a knife to her throat and MAKES her sing a song to him. If you read the tale behind the song, it's relevant. The Florian and the Jonquil. He does NOT kiss her. at all. Sansa recollects later that he kissed her - she is an unreliable narrator, and still has a LOT of evolving to do.
That scene alone would have scared ANYONE out of going with him.
2) Outside the window, WILDFIRE IS RAGING. Sansa sees that. It's terrifying.
3) She thinks Stannis has won - she has always thought if he won, BARRING SIR ILYN beheading her, that she will be safe if he wins. but hey, Stannis is one mean son of a gun and determined.
4) In the end, Sansa decides, better the devil i know than the one i don't, which is escaping and hitting the road to who knows where.
(but i would have gone because i adore THE HOUND.)
And to clarify something if you have read book three, and i won't give spoilers here. What The Hound says about Sansa, although brutish, is done to incite another to kill him. He would NEVER have done what he says he wished he had done. and besides, it was to keep her out of the hands of the Lannisters anyway, who he despises.