Yes and no. Look at the tenses of the relevant Greek in I John, especially chapter 2. We all sin, true, but all the same a person cannot be habitual in some sin, they cannot continue in some sin without remorse and without repentance, and call his or herself a Christian. Yes, a Christian may lie or steal through weakness, but a person who habitually lies without feeling any need to repent of it and get right with God (and then doing so) cannot BIBLICALLY claim to be a Christian. A person who makes a lifestyle out of stealing things from other people and continuing in that sin cannot rightly claim to be a Christian. Likewise, homosexuality - which is an ongoing lifestyle - basically proves a person to not be a Christian. Remember, by their FRUIT ye shall know them - fruit is something that develops and grows over time. If grace has to be stretched to cover ongoing, habitual, unrepented of sin, then no grace has been had in the first place.
Without repentance, there is no salvation (Acts 3:19).
It follows, therefore, that it depends on the individual sinner (as indeed it should). If someone is a compulsive liar, or kleptomaniac, or alcoholic or whatever, but realises that what is being done is wrong, then they can still be in a state of grace. Succumbing to temptation means you have committed a sin, but this is not an uncommon event with any Christian. We are all, as is said, “works in progress”.
I agree, what is more dangerous is when someone sins, but does not recognise that what is being done is a sin. If this is done out of ignorance, that is not so bad. If it is done out of some desire to justify their behaviour, that is far worse. When it is a deliberate flouting of God’s commandments, that is the sin of pride and self-will. That is, actually, what is most wrong with the “gay” lifestyle - this idea that they know how to live their life better than God does.
However, as I cannot see into the heart of each and every gay, I would be wary of making a blanket statement. Sure, unreconstructed homosexuals are in a state of sinful rebellion, but then so are unreconstructed hedonists, or unreconstructed humanists. Who can say which is worse? Personally, I just pray for all of them :)