To put it simply, it was not a part of God's original design for marriage. Marriage was intended by God to be a lifelong commitment between one man and one woman. After the fall, people began to depart from this design, which resulted in an abundance of sexual immorality. As homosexuality differs from this design, it is sin. There is obviously a lot of division around the topic, so I won't go further than this unless you want to learn more. But the Bible is pretty clear on the issue regardless.