What is your reaction when you find something that truly offends you, even though it's legal?

I would only try and do something if minors were exposed to it, other than that just simply ignore it, if it was a business certainly not patronize it, but that is all I would do,how about you?

Depends on how offensive it was. More than likely I’d inform all my media friends and let them have at it.

I would not think it meant that somebody owed me money.

I would not seek legal redress.

I would not post anything anywhere.

I don’t believe that the world owes me protection from anything I might disagree with or find distasteful.

Rather, I would avoid exposure to it if that’s possible and do my best to ignore it if it isn’t. I might gripe privately to family, though.

If it were hate speech in a public place, I would report it and try to get it taken down. Other than that, I agree with @Jeruba‘s response.

Hell I just take myself away from the situation if I can.
Every so often I go into my “other” messages on FB and just laugh at one that I’ve let sit there. Some woman who I don’t know wrote to me “That offends me. Take it down.” Um. No.

Be specific. It could not be anything. What is it that offends you?

The reaction must vary with the circumstances and must surely vary with the nature of the offense. I mean you can wake up one morning to the news of Trump’s election or be waiting at an intersection with a car full of little kids while a naked man with an erection casually crosses the street in front of you. The only reaction I think common to both examples is disgust and possibly irritation at the requirement to explain that disgust to the little kids around either incident.

A naked man crossing the street is not legal. OP specified legal offenses.

If my neighbor painted their house a repulsive color, I would grumble to myself about it.

If it were an offensive billboard, I would try to have it removed.

If someone planned to open a strip club in my neighborhood, I would do what I could to prevent that.

This is just life.