How bad is it to read other people's mail through the envelope?

[According to Pliny]

Some times of course, you just can't help noticing that an envelope says "Final Demand Letter" with a lawyer's return address. The whole point of that was to embarrass the recipient.

But I've also discovered that if you xerox an envelope with a high contrast setting that the letter inside will show up like the envelope was clear. Unless it has that privacy patterned envelope like checks come in.

Today I saw a co-worker's envelope coming from "___ Drug Testing". The thought flashed through my head he might be in rehab. So my curiosity overcame me. But he is only testing a sleep apnea drug. I was relieved for him and then ashamed for myself.

But can you quantify the badness of it? If I were a Catholic, would I have to say two Hail Mary's or get mineself to a nunnery?