Boys on the Side
The friendships in "Boys on the Side" challenge social norms in three ways. FIrst, there is the presence of a character whose sexuality does not meet social norms, which prompts viewers to think about how that changes the dynamics- it opens up new avenues for the characters to explore and discuss, ones that might not have been there if all the women had been straight. Next, there is the close bond that is formed almost immediately between the women, which is in part brought on by the harrassment and subsequent death of Holly's boyfriend. Finally, at the end of the film, when Robin figured out Jane's feelings about her, it caused the dynamic of the friendship to shift and brought them closer together, allowing them to have a deep (nonromantic) love for each other, which is not a situation that is commonly addressed in film.
The new family system challenges the social structure most obvisouly because there are no men. While there are men in the film, and while two of the women become involved with them, the dynamic of the "family" does not change. The women are the foundation of the family, and the men freely float in and out of their lives in a way that does not threaten or challenge the system that the women have created. The "family" thus becomes single-gendered, forming a relationship that is almost entirely dependent upon itself (i.e. each other).