There's so many interesting aspects of this thread I'm going to have difficulty being concise -- its also 4:30 in the morning -- but here goes. My first post too! Nothing will go wrong with this plan...
The inquiry is about what distinguishes very close friends from lovers, husbands, wives...
Probably what makes it confusing is that there are multiple spectrums of measure involved. We use the word intimacy in multiple ways and try to use context to know which is meant, but we sometimes combine them. There's:
Emotional intimacy: ranging from 'I never share anything I'm feeling with this person' to 'My every feeling is available to this person'
Activity intimacy: ranging from 'Regardless of the activity I don't want to do it with this person' to 'No matter what the activity it will be fun with this person'
Physical intimacy: ranging from 'I won't ever touch this person' to 'I'd never miss an opportunity to be physically close'
Sexual intimacy: Not sure I have a good sense of the range on this one.
There's probably others.
So for me a friend is going to be someone whom I have an activity, emotional, or physical intimacy with. A lover is someone I have sexual intimacy with. Sometimes these land on the same person, sometimes they don't. I have friends are solely 'activity friends', I have two friends whom I have a strong emotional intimacy with...and so on. Each person I have a relationship with is going to have different degrees of each of the intimacies. And it evolves and goes back and forth on each spectrum as the relationships change.
There's also another component here -- for many "friend" is simply someone they know. My view is that unless there's something going on with at least one of the intimacies, that the person is an acquaintance.
I think the intimacies feed into each other but they're separate needs and are fulfilled in different ways. For instance, emotional is fueled by talking about feelings, goals, life choices, and the day-to-day joys and frustrations. Physical intimacy can make some of that sharing easier and that sharing can make physical intimacy easier.