Nature draws men and women together for a purpose other than friendship. There are a few exceptions that totally eliminate the likelihood of attraction.
If you first meet a woman as the girlfriend of a good friend or relative, your sexual antennae are also turned off, and you might discover that you cannot even become attracted to her if she later becomes available.
If you meet a woman with a husband or boyfriend but have never met her guy, all bets are off. Harmless attraction and flirting can end up leading to more.
Of course, there are exceptions to every rule, and a lot of time alone together can also allow the forces of nature to overpower our civilized instincts. Men and women can be friends in groups, but if there is a reason they should end up getting romantic, it is usually not a good idea for them to hang out together too often without other common friends around in order to keep a platonic “feel” to the situation.
Even if they are both unattached, there can be trouble. There is a high likelihood that one of them will come to desire more than friendship from the relationship while the other will not.
Male-female attraction is a powerful force that can be difficult to keep in check when there is nothing or no one to enforce social boundaries.