I think "Middlesex" by Jeffery Eugenides might be a good one for that. Firstly, it's a wonderful book; you also have common themes such as developing sexuality in a young person (main characters are around the same age), religion and the church, family, leaving home/exile because of difference, search for identity. And there are enough differences (gender, culture, sexual difference, history ... ) to offer a lot of contrast.
I don't know if you have to pick a different author, but perhaps it would be an idea to look at another book by Winterson, as the exploration of sexuality is a thread running through most of her work, although in very different ways. "Written on the Body" comes to mind. "Sexing the Cherry" might be a bit too difficult, although it certainly offers a lot to examine.