What if:
1. A potential sexual partner tells you that they have been tested for STDs and been cleared, and have not engaged in any unsafe practises since being cleared.
2. They are lying and they in fact know that it is highly likely you will get an STD if you sleep with them.
3. You have sex with them on that basis.
4. You get an STD.
Do you think you have a basis to whine and/or sue?
It seems to me that you have some weird kind of sexual ethic going on where if you ever trust anyone enough to engage in an unsafe sexual practise, then even if the person you trust is deceiving you you still deserve whatever you get.
Maybe that makes sense if we are talking about hooking up with strangers in bars, but I don't think it does if we are talking about long-term relationships.
Step 3 will never happen. I have never had unsafe sex, and I never will.
The sexual ethic is that it's
stupid to not use condoms*. "Long-term relationship"? So what. The virus doesn't know that, or care. So you've been seeing each other for a while? So you feel True Love Brand Love? The virus doesn't care. It's either there or it's not, despite what you hope, believe, or feel. And you only have to be wrong
once. Why take the gamble? People lie, people cheat, people betray. People can be infected without knowing it. People can know it but be in denial. Why take the chance, when the stakes are so high?
Given that it's so very, very easy to just use a condom, I don't see why not.
*Unless you're the married-to-each-other, monogamous, lifelong marriage, no swinging, no cheating, tested-periodically couple that doesn't do needle drugs or get transfusions. You might consider these to be reduced risk circumstances, with some justification.
eta: Oh, and to answer the hypothetical, supposing Step 3 did occur:
No, I'd have little basis for complaining about my own stupidity. The other party would be culpable for deception, but that doesn't erase my own responsibility for taking care of myself.