How can a gay be straight?

No, they can't. Homosexuality is genetic, not a choice. Though some would disagree with this and say that homosexuality is a choice and given the right circumstances (for example, abuse or a bad relationship with the opposite sex) a heterosexual may become gay, thus if they can choose to become gay they can then choose to become straight again. There is "gay therapy" that is offered to homosexuals in order to "cure" them of their homosexuality.