Is there such a thing as love at first sight? I think it depends on how literally we take the word 'love' here. If 'love' here means no more than a feeling of romantic attraction, then there is certainly love at first sight. However, if 'love' here means, well, love, then it is not clear that most of the cases that people would describe as 'love at first sight' are really love.
When someone describes his first encounter with his beloved as 'love at first sight', it is usually a retrospective description. It is in the light of his love for her now that he sees the first encounter that way. If after the first encounter their relationship had not developed further, that would not have changed that experience, but he probably would not have described it as 'love at first sight'. So it is more reasonable to think that the first encounter was not love at first sight; it is just that it is seen as love at first sight retrospectively.