This is a complex topic and I feel it cannot be explained in one paragraph. The reason is, that it depends. It depends on the culture of the individuals. Are they from a Western culture like the US or from a European or Middle Eastern culture? The culture and family I was raised up in (European) was quite religious, so there were certain unspoken 'taboos.' For example, my late husband also had certain beliefs similar to my family's, so having a male friend while married was frowned upon. However, I have had many male friends throughout my life - before I married and after my husband passed away - through my employment, community service, church, and playing in orchestras. I treated them as equals, and platonically. There were never any romantic inclinations (and if I sensed something was brewing, I would cut it off, especially if I wasn't interested).
In addition, when one sees people as not male or female, but as friends, it takes a certain talent or type of person, as I read in a previous post here. Probably religious leaders have that special ability. On the opposite end, there are people who see other people as sex objects. I, for one, can spot such a person a mile away, by the way he looks at a woman, the way he talks and flirts, etc. I have seen couples having children together, and never marrying, and the guy goes off to have a child with another woman. Still not married with the second woman. To me, that guy is seeing these women as sex objects. On the other hand, there's the other extreme, and that's our new social media society where we build "friendships" online, never having seen the other person face-to-face or had a cup of coffee with them, yet we call each other friends.
Finally (sorry about rambling), when a man or woman is married and they have a friendship with the opposite sex outside the family, if they start relating to the other person through emotional intimacy (there's more than one way to be intimate), and sharing their secrets and dreams with them, I feel something is being taken away from the marriage. These are my thoughts. I apologize for the length of this post and if I offended anyone. Just remember, we are all friends here.