The Last House on the Right
Like everything in Cal's life, the word Middlesex has dual meanings, the obvious one being the fact that Cal is between sexes. (Insert as much talk about genitals as you're willing to stand right now.) He is both male and female anatomically speaking, and occupies both genders over the course of his life.
However, Middlesex can also mean the house that Milton buys and raises his family in. Cal refers to the house with an almost worshipful reverence. "Middlesex!" (3.3.61) he says, complete with exclamation point. "Did anybody ever live in a house as strange? As sci-fi? As futuristic and outdated at the same time?" (3.3.61). Clearly, Cal thinks the house is pretty cool.
Middlesex doesn't have doors; it has weird sliding barriers to section off different rooms instead. It's full of peepholes, shelves, and secret passages, too. Frankly, while we'd love to live there, it's totally the perfect place for Cal. How do you think the house mimics Cal? What secret passages does he have? (Yes, we're talking about genitals again, guys, and then some.)