What is the significance of the incident in the bar?

In the incident in question, Mira escapes being gang-raped in a bar after Lanny deserts her. The significance of this traumatic episode is that it effectively forces Mira to seek security in the sanctuary of a traditional marriage. That Mira is coerced by the threat of rape into such an arrangement illustrates the vast discrepancy in power between the sexes, a common theme of this powerful work of feminist fiction.
Some commentators have argued that Mira's reaction to her ordeal is somewhat excessive, not to mention thoroughly antifeminist. However, Mira's search for emotional security and stability in a hostile, patriarchal society is vindicated later on in the book when the daughter of a close friend is raped. The young lady is the same age as Mira was when she narrowly escaped being raped herself, and this appalling incident, as well as the aguish it causes the victim, has destroyed the social fabric that had bound Mira and her friends together.