Good. See, they provide a perfect solution to your situation. So what's the problem with that? Are you honestly so obtuse you can't see the other side? You can't understand that there are privacy concerns and that until you sign that form they can't just give your books to someone else in your household?
Just because you and your husband are taking out innocuous novels and don't care if your spouse picks it up for you doesn't mean everyone is in the same situation. The library, like every other institution, has rules to protect personal information and privacy. I can tell you of many examples I've seen in which someone wouldn't want their spouse picking up their books without their consent. You've both now given your written consent and are now free to pick up each other's books. Again, where's the problem with that?
Privacy doesn't have to extend to potentially volatile subjects like divorce or abuse. Some people might not want a spouse to know they are "wasting" time reading harlequin romance novels. I have often seen people on the subway when I've visited New York who are reading books with a brown paper cover over them because they (for whatever reason) don't want people to know what they are reading.
And not everybody who shares an address are a happily married couple who blithely share every aspect of their lives. Should children have access to material their parents are reading (like books about impotence or other sexual matters)?