The problem with modesty and consideration is the same.
Whether you're talking about everyday dress, using polite language, breastfeeding in public, PDA, etc., why is it difficult to behave in a manner that is considerate and appropriately modest?
In public, one should use polite language rather than graphic profanity- agreed? It's modest, polite, will not make others uncomfortable, and shows that you have respect for those around you.
Extend the same principle to the way you dress- cover the things that should be covered, and dress appropriately for the place you're going. Do not dress in a way that will make those around you uncomfortable.
Whether breastfeeding or at a water park or in any other situation where you may be less dressed than usual- be aware of what you're "showing", and be considerate and understanding that our societal norms dictate that some areas of the body are private, and that includes a woman's breasts. Feed your baby if you need to, but be considerate of those around you and realize that "flashing" them does not need to be part of baby's dinner routine. Discretion and modesty really isn't so difficult. If you're at a LLL meeting or in your doctor's office, the standard for modesty may be a lot more relaxed than when you're in a public place, but just be considerate of your surroundings.
PDA's- whether gay or straight, young or old... know what's appropriate in public and what is more intimate and should be shared in private.
Consideration and common decency really shouldn't be that difficult for anyone to master.