No one loves without conditions. [...] Love for a woman [...] is always conditional on her beauty. That, [...] and sex.
-- Nina's Mother (Part 1 paragraph 2-5)

Importance: This quote introduces the theme of conditional versus unconditional love and sets up Nina's ideology which she inherits from her mother through this conversation. Additionally, this quote emphasizes the importance and power of sex and sexual attraction in male/female relationships. Both of these themes will recur and feature prominently in Nina's relationship with Seth. With this ideology, which Nina adopted at the age of fourteen through her mother's words, it is easy to see why Nina later accepts that sex is a natural condition of Seth's love for her.

Maybe they're not "real" orgasms, since they're always when I'm alone. It's like that question: if a tree falls in the woods and no one hears it, does it made a sound? Probably solo orgasms don't count if a boy isn't there...
-- Nina (Part 1 paragraph 197)