I agree with Pierced, that would be a dick move. If you are unsure about the chemistry in person, don’t make an appointment and wait to see how you feel when you arrive. In this setting even a 24 hour notice of cancellation is still not smiled upon. Keep in mind that im sure said lady has already invested a great amount of time in correspondence with you. To say that you may change your mind if your dick gets hard in the bar at another girl would be quite distasteful.