Before answering this question, it must first be reiterated that God does not condemn people to hell on account of “being gay” - a common and misleading accusation. But I don’t have the answer to the particular question of why God permits the particular temptation to homosexuality, even though homosexual relations are indeed always sinful.
But I do know that all humans have inclinations to particular sins. Some of us are tempted to lust, others to pride, still others to anger. Absolutely no one is immune to being vulnerable to certain specific temptations.
Why God permits each human to experience particular sinful temptations is a mystery. That’s the bad news. But the good news is that all of us can overcome our sinful inclinations with God’s grace. Indeed, all of us are called to overcome our particular sinful inclinations, so that we may be made perfect in Christ.
Certain sins receive less condemnation than others; humans tend to condemn certain sins more than others because of the human tendency tend to attack sins we don’t suffer from. The lecher rarely argues against lust; the thief does not inveigh against coveting. (Sins such as greed are rarely argued against anymore, because the poison of greed has seeped into the well of modern society.)
So those who are “gay” tend to receive more attention for their inclinations than those inclined to envy or lust or greed, because the issue is a hot political topic and because most people are not subject to that temptation. This may be horribly unfair, but it is part of the human condition.
Why does God allow particular individuals to be more prone to certain types of sinful behavior? The simple answer: I don’t know, and no one except God does. But it is also true that if God permits His sons and daughters to be tempted, He also gives us the tools to overcome the evils we face.