I'm never offended by a man looking, and I ignore catcalls, as is the only proper response by a lady to such things. Responding only encourages it anyway.
You are correct in assuming that women do dress to attract attention, that's kind of the point. What is confusing for men, is that we don't want to be objectified. There is a balance between looking attractive, hot even, and dealing with the attention.
For you to look is fine, for you to whistle is fine, just don't call us names when we don't respond. If you approach us and are rebuffed, then you should say something like "sorry to have bothered you." and walk away. That gentlemanly act is uncommon and will often get you a "oh... wait, i'm sorry, etc etc etc" type conversation from the woman.
As long as you're a gentleman, you may play the game nicely. Women who treat you like dirt because you looked at their intentionally low-cut blouse are not worth your time.
The uncertainty is why it's fun to play this game. What spoils the fun is flirting and then being expected to "pay up" with favors, money, etc because he/she bought you a drink or something. Don't take it so seriously and you'll have lots of entertainment.