Depends. There are times I am flush wioth cash and pay. There are times when I have a brood of nieces and nephews and I pay a portion. There were times between jobs where I skipped admission and walked in.
Not paying is nothing to be ashamed of as your tax dollars are actually subsizing the museum. However, if you can afford the admission price, you are helping to subsize some of the less affluent people who visit and can't pay.
Given the great disparity in income between Super-Rich and Rich and then between the Rich and everyone else - I have no qualms about letting the museums rely upon big-deal donors to pay the way.
Selfish? maybe. Realistic in this world rich and super rich? yup. Any guilt about it? none.