Many married women (and married men) insist that having a best friend of the opposite sex is perfectly healthy. In fact, they say that opposite sex friends make better friends because they bring very different perspectives to the relationship. But let's look at a few things here.
First, healthy friendship involves emotional intimacy, as well it should. Deep friendship leads to a level of sharing that is selective and usually confidential. That means, others are excluded from the conversations. When a woman shares intimate feelings with a man who isn't her husband, a wedge forms between her and her husband. He is excluded from the privacy she shares with her male best friend. And when this starts to happen- beware. Husband is on the outside looking inward.
Second, let's be adults. Physical intimacy is the sequel of emotional intimacy in most healthy relationships. That's the way we are wired as humans. Give emotionally intimate heterosexual couples enough time and physical intimacy follows. Or, at least the temptation to be physical emerges. In same sex friendships between heterosexuals, natural boundaries exist preventing sexual intimacy from occurring.