Why do some Christians believe it is moral to be a homosexual?
This answer presupposes that homosexuality as referring to the sexual orientation per se, not the sexual acts of homosexual persons.
Homosexuality is a gift to be celibate
In the Scriptures, it is recorded that Jesus spoke about natural eunuchs (Matthew 19:12). First century contemporary evidence as well as ante-Nicene evidence shows that these eunuchs by birth have no desire for heterosexual marriage due to what we know today as homosexual orientation, intersex conditions as well as transsexualism.
Philo ,a first century Jew (1st century), knew that a man who is natural eunuch does not have sexual desire to a female ( Philo, On Joseph, XII. 58-60).This shows that the eunuch by birth in Matthew 19:12 means men who does not have sexual desire to a female.
Clement of Alexandria provided a complementary perspectiveabout the born eunuch, by way of quoting the Basilidian Christians with respect to the gospel verse about eunuchs (Stromata 3.1.1):
Some men by birth have a nature to turn away from women, and those who are subject to this natural constitution do well not to marry. These, they say, are the eunuchs by birth.
Strong's Concordance:2135 eunoúxos – properly, "alone in bed" (i.e. without a marriage partner) – literally, a castrated (emasculated) man; a eunuch.[ HELPS Word-studies]
Emasculated (ευνουχίζω) = deprived of masculinity [WordReference English-Greek Dictionary © 2015]
How does that translate to natural eunuchs, to eunuchs born that way?
Because the context of Matthew 19:12 is about heterosexual marriage, it highly implies that this shows that eunuchs born that way have "no sexual desire to the opposite sex." It fits the definition of eunuch as emasculated or deprived of masculinity. This may be what we call today as homosexual orientation.
Coming from the same literary source, the issue of divorce in Matthew 19:1-10 and the discourse on threefold types of Eunuch in Matthew 19:11-12 interpret each other.
In Matthew 19:1-10, Jesus spoke of divorce in the context of heterosexual marriage while in Matthew 19:12, Jesus spoke of eunuchs who cannot undergo heterosexual marriage.
Jesus spoke of being “one spirit” with his church in the context of heterosexual union, speaking of “one flesh” fact (1 Corinthians 6:16-17, v. 16 cited Genesis
Same Sex Relationship in the Bible is not sexual but spiritual. It is living in holiness.
David and Jonathan’s relationship is an archetype of the relationship of Jesus and his church.
1 Samuel 18:1b Jonathan became one in spirit with David,
1 Corinthians 6:17 But the one who joins himself to the Lord is one
spirit with Him.
1 Samuel 18:1c and he loved him as himself.
He who loves his wife loves himself.
Persons who are attracted to the same sex are not sinning. Their lack of attraction to the opposite sex is moral. The Lord Jesus himself spoke of them as blessed for the gift of celibacy (cf. Matthew 19:12). The relationship of Christ with the church is a parallel to the spiritual same sex relationship between David and Jonathan.