Homosexuality is a socio/cultural construct. Biblically speaking sin is regarding a person's conduct. For example, I am attracted to the opposite sex (and that's not a sin), but if I act on it outside of marriage, then I'm sinning (that would be adultery).
God is more interested in shaping our character rather than just fencing in our actions. In the Decalogue (Exodus 20:2-17; Deuteronomy 5:6-21), after the commandments against adultery and theft come the commandment(s) against coveting.
Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
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Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Matthew 5:21-22; 5:27-28
Under the New Covenant, we are no longer subject to the law, but are counted as righteous. It is nothing that we did. There is nothing we
could have done to obtain righteousness on our own ("All have sinned and come short of the glory of God" -- Rom 3:23). It is the free gift of God made available to us through faith. (See Eph 2:8).
We are nolonger under the Law. But that does not mean we are free to sin with impunity:
What shall we say then? Shall we continue in sin, that grace may abound? God forbid. How shall we, that are dead to sin, live any longer therein? Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been planted together in the likeness of his death, we shall be also [in the likeness] of [his] resurrection: Knowing this, that our old man is crucified with [him], that the body of sin might be destroyed, that henceforth we should not serve sin. For he that is dead is freed from sin.
Romans 6:1-7
I don't believe that there is any disagreement between gay Christians and fundamentalist Christians on the above. It is only over the specific issue of exactly what the so-called "clobber passages" forbid. (And whether we are still under the Law for those actions)
Being gay does not excuse sexual immorality, but sex is not in itself inherently immoral; it can be proper when done in the right time and place with the right person -- with a covenanted spouse. And this is true whether the spouse is the opposite sex or the same sex.
And Paul gives us specific instructions on what to do when Christians honestly disagree on whether an action is sinful or not: those who believe it is not sinful are not to be a stumbling block for those who believe it is a sin and who are fighting the temptation. Those who believe that it is a sin are to leave their brothers to God's judgment:
Who art thou that judgest another man's servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand.
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He that regardeth the day, regardeth [it] unto the Lord; and he that regardeth not the day, to the Lord he doth not regard [it]. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks.
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But why dost thou judge thy brother? or why dost thou set at nought thy brother? for we shall all stand before the judgment seat of Christ.
Romans 14:4, 6, 10
It is quite likely that calling oneself a Christian when one is not is, indeed, a form of taking His Name in vain (see Matt 7:21-23). But that is between them and God.