Why is it so important to people that the guy should be the taller one in a relationship?

When the man is taller than the woman, it shows a sense of - as you stated, "dominance" and also, in terms of relationships, that he is the one that keeps the woman "safe" in any retrospect (money, job, life, etc.). Much like how some men like to see their girlfriends wearing their clothes because their shirts are so big on them, it gives a sense of what would be called, "manliness" or how a man is supposed to be. When the female is smaller than the male, not only does it portray an image of him but more also her because women in media and certain cultures, females are suppose to display femininity by being smaller than the man either in size, shape and/or weight. When the man is shorter than the woman, most people come to the assumption that she is carrying him throughout their life together rather than vice versa. The woman being taller than the man can also display female dominance which can be intimidating to some men. Height differences in male and females and the meanings we as human beings place on them can be traced all the way to what we know of the dawn of our species. You don't hear of too many giants being females (e.g. Goliath) and the only mention of female giants are the Amazonians and of course, most people think of them in purely a sexual sense.

I honestly think that it doesn't matter, in fact, my crush is the shortest kid in my class, only a few cm shorter than me and I don't mind. :)