To explain the stumbling, keep in mind that he’s probably never kissed anyone before, ever. We understand this because he fell in love just on the cusp of puberty and carried the torch up until Harry disillusioned him in the wallway after saving Lesath. Felthorne doesn’t have the context we do to know the depth of the situation, but she realizes that she is his first kiss just before being obliviated, simply from the physical details.
To explain the stumbling, keep in mind that he’s probably never kissed anyone before, ever. We understand this because he fell in love just on the cusp of puberty and carried the torch up until Harry disillusioned him in the wallway after saving Lesath. Felthorne doesn’t have the context we do to know the depth of the situation, but she realizes that she is his first kiss just before being obliviated, simply from the physical details.