The difference between we think of it as normal and we have come to think of it as normal is that the second one says that this type of thinking began in the past, and underwent a somewhat gradual process.
I have come to understand modern art.
(I didn't always understand it, but now I do.)
This is like saying: I have become an understander of modern art, or in your sentence: We have become poeple who think that it is normal.
- For Teachers