Universal Forces by Monica Shaughnessy
Cassie, the daughter of an astronomer, falls for Jake, a boy brainwashed by religious zealots. Their relationship is further complicated by the fact that Cassie's best friend, Daniel, is gay, and Cassie is terrified to admit it to Jake. If she does, she might lose him. If she doesn't, she might lose Daniel.
Far from a bland "gay sidekick," Daniel's sexual orientation provides the impetus for several story turns and complications, and he lives out a very rich subplot of his own, including mustering the courage to take a date to prom and dealing with a questioning football player who's desperate to maintain a heterosexual facade.
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