Here's an SAT analogy for you. Dannie:Richard::Genevieve:Carol. In other words, Genevieve and Carol are very similar. Genevieve makes rude comments about Therese's youth, and it's clear she's attracted to Therese. And she's a babe.
Therese is flattered to be flirted with, but she realizes that she doesn't want Genevieve—she only wants Carol. We're not sure why, though, and maybe Therese doesn't know the specific reason, either. As Dannie says, "I think friendships are the result of certain needs that can be completely hidden from both people, sometimes hidden forever" (10.121). Therese and Carol can't put into words how they feel about each other, but they know it feels good.