Bingo!
The analyses here are right on, except for one tiny detail. The "protected" classes are arbitrary and capricious; to say nothing of absurd.
A 300-pound slob male can come to work as a woman and since the law "covers" him, it's OK.
I could live with a totally unrestricted right to work for someone, or not; and to employ someone, or not.
The analysis here applies simply to losers and the marginally competent. Where I work, I am sure if any smoker is singled out and fired, every smoker would quit.
Incidentally, the marginal employees, the chronicaly absentee and irresponsible are usually non-smokers. and female.
Sexual orientation is a protected class in very few places. Most places, you could fire this guy on the spot.