I have seen many relationships in my time, and my conclusion (subject to change) is that unusually close male-female relationships that are nonsexual don't work very well.
One exception is when the people have known each other since early childhood and their relationship is more like siblings.
Invariably, I find that at least one of the two parties ends up desiring more from the relationship (as in romance) as time goes on. As a practical matter, if it's the male who grows to want more from the relationship, very often that romance will not blossom. If it's the female who grows to want more, all she pretty much has to do is make herself open and available to it and the guy will tend to follow her lead (and will bite if he is anywhere from indifferent to anxious for sex, depending on a lot of factors).
Anyway, unusually close different-sex relationships are probably more trouble than they are worth, regardless of whether they are each single, dating someone else, or are married to someone else. I have never seen one really work out as well as close same-sex relationships.
I have a moderately close relationship with an attractive female, for example, and it's been mutually frustrating for almost the entire decade we have known each other. Whenever I grew more attracted to her, she wouldn't be interested. Whenever she was more attracted to me, I'd be preoccupied with someone or something else in my life and not bite. The more time I spent away from her (and her from me) the more appealing we appeared to each other. The more time we spent closer, the less interested one or the other grows. No joke, we have been doing this dosey-do for a decade now.
An older person who knew neither of us very well saw us particularly contentious with each other at a social event (as often sexually uncultivated people can be) and casually asked someone 'how long have they been married?' When she was alerted that not only were we not married, but never even dated and were 'just friends' for many years, she laughed and suggested we both just get a room and work things out.
Old timers can be very wise, you know? :-)
PS - I now date another woman entirely and this 'friend' can't stand her, even though my companion has been very nice to her the few times they have met. Go figure...
Very True Statement as I said before look at the Harry Met Sally story it was so very true...
I have a close female friend that I have known since high school. We have been friends for over 20 years. So like you said, we are more like siblings.
Funny thing is, even though we are both married, her Mom remarks from time to time, that it is too bad that her and I didn't get together. I suffer from Eddie Haskel syndrome.