So maybe this might help some people come to grips with this.
When you agree to have sex with someone who tells you they are HIV negative, those people fall into 3 groups:
1) People who truly are negative
2) People who are positive, but tell you they are negative to get into bed with you
3) People who are positive, but tell you they are negative because THEY DON'T KNOW THEY ARE POSITIVE
So obviously, people who fall into group 1 are no danger to you. Statistically, if you look at the number of cases where someone intentionally lies about their status, it's pretty small. Look at the number of cases you've read about that in the news, compared to the actual number of HIV positive people in the US. It's miniscule.
Now the third group is the one that you REALLY need to worry about. The CDC reports that 1 in 8 Americans with HIV don't know that they have HIV. To compound the issue, since they don't know they have HIV, they aren't on medications, so their viral load is VERY HIGH, and they are VERY contagious.
The people in group 2 are much more likely to be on medication, have a low or undetectable viral load, and have a much lower chance of passing on the virus.
So, to recap, if you are using "Well, I only have sex with HIV negative people" as your sole means of staying negative, you are putting your health at risk. You CANNOT trust people when they tell you they are HIV Negative, because there is a good chance they simply do not know their status.
This is why I don't view this change in law as that big a deal, because YOU SHOULDN'T BE TRUSTING PEOPLE WHEN THEY TELL YOU THEIR STATUS in the first place!