Sorry for the rant, but...
The one thing most people never understand -- you cannot legislate morality, it has to be cultured and nurtured. Even then, some of the most supposedly moral and respected people are practicing some of the most risky and even morally dispicable, sexual behavior.
Wanna get a real good shock? Ask a bunch of kids (say, any group of people 21 and younger, and even much, much younger) if there are virgins among them. Quite a few, and many more than you would assume, will say that they are. However, then ask them what sort of things they and their friends do when they're making out. Be prepared to pick your jaw up off the ground.
Next, ask any group of people of almost any age how they think STD's and HIV can be transmitted. These diseases are spreading among groups you wouldn't "traditionally" expect, but you won't know about it unless you read the health statistics. The general public is not getting the idea about how bad and how widespread these diseases are. "Nice girls don't get pregnant unless they are married," and "Nice people don't get STD's" didn't work in the 50's, so what makes anyone think it'll work now? Maybe there are really that many "nice people" around, or maybe they really aren't so nice after all.
People of all ages still assume that unless you are homosexual, you cannot contract HIV. They also assume that only intercourse can cause the spread of STD's. They assume that you cannot get pregnant unless you have sex many, many times. There are many people who view themselves as virgins and non-sexual even though they practice many types of sex. Few, if any think it can happen to them. Want to see someone with a blank, deer-caught-in-the-headlights look? All you have to do is then ask 'em, "Why not you? Are you someone special with a huge medical freebie pass?"
Almost no one knows for 100% certain that their partner isn't cheating, hasn't used IV drugs, hasn't practiced risky sex (and, really, it's all potentially risky), or whatever? Very, very few people do, but many think that they do. That means a lot of people willingly trust others who lie to them or would otherwise say anything, with their health and well being.
No law or rule will stop these diseases. Ignorance and assumptions will continue to allow the spread of infection. Until people start to respect each other and themselves a whole lot more, the health of the world ain't gonna improve all that much. The only hope to stem the spread of these diseases seems to be education and respect. No laws will prevent anyone from doing what they want to do, and worse, what they think someone else wants them to do to keep them in the relationship. So, Sex Ed might not prevent kids from having sex, but it might keep them from becoming pregnant, getting sick, or even dying. So can just taling to your kids.
[END RANT MODE]
Look up the CDC and various state health departments' statistics on the spread of STD's and HIV/AIDS and really see who's getting sick.
Yes, if someone has a disease they should tell their partner -- before, preferally than after. No law will make that happen. Only respect and caring will work. Since people still think that wrestling is for real, we could be in trouble.