Why are people gay? is there a chemical imbalance?

No: According to the american psychiatric association there is nothing wrong with gay people. Is just a preference. Some people doesn't like fish, others doesn't like the purple color. Their sexual preference goes to people of the same sex.

It's genetic: Although there would seem to be no evolutionary advantage to homosexuality, heterosexual parents in all places and all races continue to have homosexual children at about the same incidence generation after generation; so it seems to be part of the human condition. In view of this modern knowledge, judgments about the "abnormality" of homosexuality tend to lose force.