Commonly women have to conciously choose a guy that is not top 5%, because they have to consider the risk of the man having too many temptations to go astray.

The main real exception is when they are also equally attractive and know that their boyfriend/husband uniquely has it for them.

How common is it for really attractive women to marry really attractive men?

EDIT: Why is this true? Because of the infamous 80/20 rule spouted here so often. 80 percent of the women going after 20 percent of the men. So what happens to the men in the 20%? They get inordinate amounts of attention. Women who fawn after them. Women who chase them. When this happens year after year after year, it becomes a huge temptation. Normal dudes just don't understand the magnitude of this issue.

Why do certain men have harems or women who share them? Traditionally women have seen this with their own eyes- other women trying to steal their man. That is why a woman can be wary of marrying a guy like this? What are your thoughts?

Many average men here say that they can't get female attention to save their lives. Women often pick men because they know that they don't have other options. That's why unattractive couples work so well. Neither party has outside options, generally speaking.