To me it seemed while the kiss caught Sansa off guard, she rolled with it. She didn't grimace or pull back, she closed her eyes and went for it. Out of context it looks just like a normal first kiss between two people with mutual feelings for each other.
It will be much more interesting if Sansa falls for Baelish and agrees to marry him willingly, as opposed to her being forced into it and slinking back into to that listless hostage role.
At first I thought killing Lysa like that was an odd move since Robin retains claim to the Vale, but maybe that's how Littlefinger wants it. Tywin has proven time and again that influence can be every bit as effective as direct ownership. Like Tommen, Robin is little more as a figure head. Baelish is second in line, but holds all the power. If his ultimate goal is the Iron Throne, it doesn't matter if Robin is Lord of the Vale, in fact it's probably better to have someone who's supremely loyal to you in place as a ruler of a major territory.