Question: "What does the Bible say about bisexuality? Is being a bisexual a sin?"
Answer: The Bible nowhere directly mentions bisexuality. However, it is clear from the Bible's denunciations of homosexuality that bisexuality would also be considered sinful. Leviticus 18:22 declares having sexual relations with the same sex to be an abomination. Romans 1:26-27 condemns sexual relations between the same sex as abandoning what is natural. First Corinthians 6:9 states that homosexual offenders will not inherit the kingdom of God. These truths apply equally to bisexuals and to homosexuals.
The Bible tells us that a person becomes bisexual or homosexual because of sin (Romans 1:24-27). This does not necessarily mean sins the person has committed. Rather, it refers to sin itself. Sin warps, twists, and perverts everything in creation. Bisexuality and homosexuality are caused by sin "damaging" us spiritually, mentally, emotionally, and physically. Sin is the plague, and bisexuality is simply one of the symptoms.
Many Christians mistakenly focus on bisexuality and homosexuality as particularly evil sins. The Bible nowhere describes homosexuality as being any less forgivable than any other sin. A bisexual is the same number of steps away from salvation as the "moral" legalist—one. God offers forgiveness to anyone and everyone who will trust in Jesus Christ for salvation. This includes those involved in bisexuality. Once salvation through Christ is received, God will begin the process of destroying the acts of the flesh (Galatians 5:19-21), including any and all homosexual tendencies, and developing the fruit of the Spirit (Galatians 5:22-23). The promise of a "new creation" is available to anyone who will trust in Christ (2 Corinthians 5:17).