This is a list of Japanese idols; a type of celebrity in Japan. The word "idol" is almost always used to refer to a young woman, although there a significant number of male idols. The following list includes both female and male idols as well as both solo idols and idol groups.
Male idols are usually referred to as idol groups.
Male idols are male idols where as more than one would constitute a group whether male or female is what I infer from the snippet. I think the ai got confused because they are both mentioned in the second comparison of each part of the last sentence.