This means either couldn't get it up, had a spongey boner, or was hard and then he lost his boner. Or some other aspect where it just doesn't work and you can't have sex.

Does it happen rarely? Or is it more common than people would think?

Is it a complete dealbreaker for you if that happens the first time or would you give them another chance? If you did give them another chance, how many chances would you give before you decide to part ways?