Never. I would never intentionally bend something that did not belong to me. I don't even dogear my own books.
I abhor any sort of damage to my books---I'm really weird like that. Maybe it's because I love buying/owning old books but I don't like them to be damaged. Who knows?
Speaking of husbands, though, just the other night I noticed my husband was reading (as is usual) but also had a highlighter in his hand. I asked him what he was doing & he held up a biography he's been reading and said he was marking passages he wanted to remember. (It is his book, not from a library.) It took me some time to process the fact that he was willingly writing in his book! It blew my mind! I could never do that to one of my books.