I would have discreetly put it in the garbage.
You are a better person than I.
The one time that I’ve seen pornography donated there happened the same day (although probably not the same donor) as a copy of “Art of the Deal.” I put that in fiction, and then showed it to the woman in charge. She wondered what to do about the pornography, and I told her that there was a solution: I broke the DVD, cut the box in half, put them in a paper bag, and threw them in the trash. I don’t care if consenting adults want to look at that, but the library is not the place for it, and minors do work there.
p.s. The lady in charge had a good chuckle, and suggested that I move it to history/biography, which TBH I did. We later found an algebra textbook in the sci-fi and horror section.