The image that comes to mind is that of a buffet, an elongated serving table (counter) with many different dishes.
People usually walk by in a line, following each other. If you don't know what dishes there are further on, you may fill your plate with food in the beginning, only to discover that further down the line there were other dishes that you would have liked a lot better.
The author compares the selection of a partner to choosing food from such a counter. By stating that the error of modern technologies is that you have to chose before walking the whole counter, the author indicates that if you find a partner using modern technologies, you are likely to find someone more suitable later on.
Internet romances make sure that the buffet where we can "pick" a partner becomes longer and longer, we have more choice, and less chance of knowing what to expect further down the line (ergo, more chance to become disappointed with our initial choice!)