not to say it's right or wrong, but there are a high number of those that have contracted a STD and then think they owe it to everyone else to give it to them.
I dated someone with herpes, I didn't catch it from her. She found out she got it at 15 from a guy that knew he had it, so she did some research and met another chick in sort of the same boat that went on a rampage... He was her first too (they both got it orally but it was the genital one), fortunately my friend didn't have much of a problem with it other than losing dates from being honest about it. I ended the relationship with her in a way because of it...looking back I feel bad about that, but I was not 35 years old then.
For a while that chick would just pick up guys for BJ's...she was a kid though and getting something like that probably felt like the end of the world.
People are judgmental, but anyone that has sex could have a kid, a disease (actually about 25% have herpes or HPV and don't realise it which is scary), etc. I have found though most that judge never really lived. Add that to the fact that most people that have or had something won't openly admit it.
HIV/AIDS is a different deal. I can't say I can speak from experience in it, but if I had it I think I would tell people. I also think I would spend a lot of time building up to that event. Outside of that I personally feel it's up to me to protect myself...no one 'owes' me any type of disclosure in a sex type relationship. I would expect it if we were becoming 'mates', but outside of that I already suspect it's a possibility.
All that said, if you are really out there having sex there are times when you are only lying to yourself if you say you were always practicing 'safe sex'. Most of the kind of people that swear they never practiced non-safe sex never really did more than stay with the first person they ended up having sex with.