What's wrong with a long-term, committed same-sex partnership?
Clearly, love and commitment are virtues and so it is tempting to see a loving, committed same-sex partnership as a godly relationship, particularly when compared against unloving or unfaithful partnerships of opposite-sex couples. However, the Bible is clear that a same-sex partnership is by its essence outside of God's creation plan that a marriage be limited to the union of one man and one woman (Gen 2:21-24). The Bible lists no exceptions to this model based on the depth of the love, the quality of the relationship or the level of commitment.
The argument is often put forward that the early church knew nothing of loving, committed relationships other than marriages between men and women, and therefore that the Bible is silent on the issue of modern-day same-sex partnerships. However, in 1 Corinthians 5:1-2 gives the example of a man in the Corinthian church who was in a sexual union with his father's wife - such a relationship is forbidden in Leviticus 18:8. There is no discussion of whether the relationship is loving and committed or not. Paul is clear to the other Christians: "Let him who has done this be removed from among you." (1 Corinthians 5:2).
Another argument is that the homosexuality outlawed in the Bible was all either abusive (eg between men and boys) or part of pagan rituals, and so the biblical prohibition of homosexual practice was about stopping the abuse or paganism rather than about condemning committed homosexual relationships between consenting adults. However, this fails to consider the widespread adult-adult homosexual relationships in Greek and Roman cultures in New Testament times, of which the writers would have been well aware. Corinth in New Testament times was a Roman colony on the Greek mainland, but when Paul wrote to the church there he reminded them to avoid sinful behaviours, including homosexual sex. In 1 Corinthians 6:9-11, he also lists idolatry separately from homosexual behaviour, making it clear that homosexual sex was distinct from idol worship.
Resources for further study
Please note that any external resources below are intended to complement the main answer given above and may not entirely match TFT's position.