“The gospel demands every believer—without exception—to forsake not just sinful actions but also the sinful passions that fuel them.”
False. We can control actions but not urges. If I lust after a married woman, I need to try to stop lusting after her, and I certainly need to NOT ACT ON IT. As the saying goes, you cannot stop a bird from landing on your head, but you can stop it from building a nest there!
ALL CHRISTIANS still struggle with sins. Which sins cause us the most problem varies. Pride and anger are mine. We have no right to justify those impulses and we need to confess and repent when we give in to them - which we all do, sometimes. Justification comes the moment you believe, but sanctification - becoming good - is a life-long process and no one has finished prior to dying.
The biblical insistence that Christians must resist temptation and so maintain sexual holiness, in both desire and behavio[u]r is very real.
The tenth commandment is Thou Shalt Not Covet. This means we should not so much as desire to sin. Nor desire good things overmuch. Purely a thought and heart issue. So yes desiring to sin sexually is indeed a sin. See Jesus telling us we can not look at another (presumably non spouse or intended) with lust without committing adultery in our hearts.
There are degrees of sin. Contemplating a homo act is not as wicked as doing it.
But it’s still a sin.