Denise has very specific—but very different—tastes in lovers.
On one hand, she often finds herself involved with older, emotionally detached men (we're looking at you, Don Armour). On the other, she has a love for bright-eyed young women that's only rivaled by Hugh Hefner. Denise manages to keep these two worlds separated for a while, but her relationship with the married couple of Robin and Brian brings her two preferences to an abrupt collision.
Although Denise isn't attracted to Brian per say, that's par for the course with her relationships with men. Denise loves to be wanted—she notes that she was "vicariously excited by" his attention, although "she didn't mistake that excitement for attraction" (5.331). It has little to do with Brian himself, in other words.
Then there's Robin.
Denise "had never wanted anything" (5.686) like she wants Robin—both sexually and emotionally. However, their dynamic as a couple begins to replicate that of her parents, with Denise taking Alfred's role and treating Robin with "a deliberate, calculated form of cruelty" (6.457). While this might seem odd at first given Denise's clear desire for Robin, coupled with her relationship with Brian, these two help us recognize just how uncomfortable Denise is with relationships in general.
By the end, it's clear the neither relationship is particularly healthy for Denise. However, her ill-fated love triangle helps her start to make sense of her identity. It might not leave her with any real answers, but it's the first step on a long road of self-discovery.