Because all of the straight Christians are not sinning?
Since God has condemned homosexuality in every scripture in every monotheist religion and some that aren’t, a person who willfully continues, without repentence or the desire to change, in homosexual behavior, such a person cannot be a follower of any religion in truth.
This also applies to people who willfully and without any attempt at repentence continue in other forbidden behaviors. But with man/woman sex for instance, there is the option of marriage; with same sex behavior, there is no way to sanctify it.