It’s because people cheat on their spouses a lot more than people think and a baby getting hepatitis B during vagina birth is just awful. Basically everything in standard of care assumes the worst about the individual.

There is a test for that ( and for heaven's sake -- doctor is where you go for awkward conversations to begin with ) and I genuinely resent that the real reason is being hidden from me as a parent. How on earth am I supposed to make decision if I am getting inaccurate information? Or more importantly, as a non-doctor, why I am I forced to learn all the things I should be able to trust my healtcare provider with? Why do I have to peel the onion for various agendas for something this important? I swear.. each day it seems healthcare starts to resemble Office more and more..

It is infuriating.

Then perhaps cheaters should be punished, and not parties not involved in the cheating?

The state doesn't track who is cheating, so all they can do to protect kids is to assume everyone is equally likely to cheat. If you think it is better they tracked your sex life to ensure they didn't punish the wrong people then you have very different values than most.

It doesn't have to be active sex life tracking, there are other ways. When cheater are punished, it will make others less likely to cheat. Win win for everyone.