She is with him under the thrall of the pixie dust up until that split second pause before he deepens the kiss. It is just long enough for her to finally get a grip on the situation. Elektra tenses as it finally sinks in that he's touching her.

She cares about Wesley. He is her friend. Undoubtedly she would cross a lot of lines to ensure his happiness and safety if it came to it. This wasn't one of those times.

She shoves him back with her free hand while the other that had been gently cupping his cheek seconds ago is getting pulled back. Elektra forms a fist and hits him hard before she can really think about what she is doing. It's a knee jerk reaction; she doesn't like being touched with very few exceptions.

After all if Elektra had been thinking instead of reacting she might have noticed they were both covered in the same tell tale gold dust.