Most conservative Christians answer Yes to that question. And same-sex-attracted persons listening feel condemned because they understand homosexuality to be their sexual orientation, just like heterosexuality is a sexual orientation.
But when conservative Christians say “homosexuality,” they usually think of gay sex, rather than sexual orientation. And that creates a lot of misunderstanding.
So back to the question, Is homosexuality a sin? And the straightforward answer is No.
Homosexuality (i.e. a homosexual orientation) is no more a sin than a heterosexual orientation. Both gay and straight person may sin sexually by engaging in sex outside of marriage, since God designed sex exclusively for life-long marriage between a man and a woman, as the Bible makes clear.
The Bible defines sin in terms of what we do or think, not in terms of who we are or how we are tempted to sin:
1 John 3:4 “Everyone who sins breaks the law; in fact, sin is lawlessness.”James 4:17 The JKV says that is is the “transgression of the law.” That has the sense of stepping across a line that is drawn, or breaking the law, as in breaking a speed limit.
Romans 14:23 “.. everything that does not come from faith is sin.” (NIV)
James 4:17 “Anyone, then, who knows the good he ought to do and doesn’t do it, sins.”(NIV)
So don’t let anyone tell you that is is a sin to be gay/homosexually oriented. Sin is something you do or think, not something you are.
That said, we are all sinners in need of saving – both gay and straight people. And that’s why Jesus came to this earth to walk among us and die for us – that all who choose to accept His saving grace and power will be saved to be eternally with Him.