I agree, if a Christian is doing something where he is constantly living in sin something is wrong.
1 Jn 3:9 Whosoever is born of God doth (present tense) not commit sin; for his seed remaineth in him: and he cannot sin (present tense), because he is born of God.
John is not saying it is impossible for a Christian to sin, but a Christian cannot continue (present tense) live in sin.
There is a big difference between committing a sin, and living in sin. It is possible for a Christian to live in sin, if they get away from the Word, and away from God, they can easily backslide to the ways of the world.
I defy anyone to live for a normal day in their life and not commit a sin. They only ones alive on earth who can do that are babies, the very mentally challenged(with IQ's the size of small children) because God holds those a different standard, or those people considered to be brain dead.
If anyone truly thinks that they never sin, I am sorry, but you are mis-guided.