Completely understand as we had a "romantic afternoon" rudely interrupted last March. Yes, the latch will prevent them from entering, but nothing will stop them from knocking and disturbing you. A natural reaction, no matter what mood you are enjoying, when you hear knocking at the door is to respond ... which can quickly ruin a mood. All because some idiot in the insurance or legal department thought this would be a good idea ... when it does NOTHING for security. Believe me, it didn't come from the security department or they'd be having actual security doing it instead of housekeeping and miscellaneous cast members. The real security knows that all it does is inconvenience guests.
You will get worlds of "no big deal" and "no issue" from the folks that think it's "security" and for those that haven't been impacted. And the truth is that the vast majority are NOT impacted, because it is done so haphazardly and depends so much on random timing. We've moved our stays to offsite because of it, but the lure is there to make exceptions in the future and return.
My best advice would be to go in as a pair with your heads set that you will hang the sign in hopes of avoiding a knock, latch the door so you won't get walked in on, and agree to completely ignore any knocking that might be heard ... above and beyond what knocking noises might arise from other activities

Whatever they might be knocking for can wait ... whether it's an "inspection", to "change a light bulb", "empty trash", "maintenance", or any of the other excuses the various cast members that have to do this have come up with.