Seems to me they could avoid offending anybody (too seriously anyway) by just having 'weight classes'. After all, isn't weight of the plane directly correlated to fuel usage, and hence, profit loss? Just have three classes: under 100, 100-250, and 250+. And they can pick some weight/height ratio limit that determines whether you have to pay for the adjacent seat or not. At least it's more dignified than having your ass measured!
I meant to say that the three different classes would each get one seat, but pay different ticket prices. If you reduce the issue to something abstract and justifiable, such as weight vs. fuel usage, it is less offensive to the paying customer. What's more, it makes sense. Children pay less at fast food restaurants. Why? Because they get less food. That, and they're loss leaders, i.e., they get the parent(s) in the store, where they too may buy food. That's the whole logic behind the 'kids stay free' or '1/2 price' things you see. People are more likely to travel with that sit-on-lap baby if the baby doesn't cost anything. But that's a different issue.
The point is, the airlines are trying to maximize profit. Period. Telling people they're too fat for one seat is effective IF people do not use that as a reason to shun your airline in the future. Therefore, it may make more sense to choose the more tactful, quantifiable route I described, or not implement it at all. Sometimes, enforcing a rule that gets you more money in theory, backfires because it drives away the customers.