Whether it be a female or male -- close friends are those you enjoy spending time with, trust, respect, and even confide in depending on the topic. Most close friends are relationships that have been developed over a very long period of time and have tested time and distance. When I was a teenager, some of my closest friends were males - and there wasn't any romanticism. Yes, many relationships do dance around the possibility of dating but the longer you live, the more you understand and appreciate that friendship doesn't have gender. Many couples have trust issues and therefore in my opinion many women and men don't friendships with the opposite sex.
If we were to take this author's position further -- doesn't this imply that if I had a female lesbian friend -- that I couldn't have a close friendship with her -- being that she's female and I'm female. I'm heterosexual and this author was speaking about those relationships assuming since I'm a girl I can't have guy friends.
The same argument would mean I should not have lesbian friends. And yet--one of my very closest friends is a lesbian.
Again -- blanket statements like this author claims hold no water. People who don't know how to enjoy pure friendships while respecting their marriage or romantic partners is an entirely different topic.