I wonder if a level of interdependence is an aspect of intimacy as well. I can't think of a relationship that is of substance that doesn't have some level of mutual benefit. I think you can have similar levels of dependency on multiple people (emotional support from close friends, for example) but both parties in each relationship must depend on the other for the relationship to develop intimacy.
To address the question of culture, also from very limited anthropological education, I believe that intimacy is generalized by each sub-culture (in our society, anyhow). What I have learned as intimacy as a white, middle class, presumed heterosexual female is different than another person's experience. However, if you gather many people who have been raised in similar homes, I am willing to bet we would describe intimacy in a same way. My first instinct is to describe intimacy as sex. "Being intimate" with someone is the jargon of Polite Society in my culture of origin. Also, I will generalize here, white middle class culture does not emphasize larger community groups as much as other cultural groups. Therefore, I have experienced less intimacy (broad definition) than others may because of my cultural heritage. As I build my own community this is becoming more obvious.
Me (Lilac) - pan, queer woman, spouse to Programmer
Programmer - Spouse of Lilac, bisexual man
Not currently in relationships with others.