How much highlighting, underlining and other writing you are willing to accept in a used book is a matter of personal preference.
Contrary to what you might think, cheating in a marriage is a result of underlining problems that already exist in a marriage.
Underlining text other than links: Underling text such has headings will make them appear to be links that don't work.
Once you know why your anger is manifesting, you can begin to address the underlining issues.
In your case, there are two underlining issues to explore, if you haven't already done so.
Choose either bold, underlining or italicizing, and only do it once or twice per page.
Another way to emphasize your keywords is by using bold, italics or underlining.