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There is something called "window period": the time from infection until the time it becomes detectable in the infected person:
http://www.safelabcentre.com/faq.html
For HIV testing, the window is two weeks minimum, according to this lab. That means that if you get AIDS today, and get tested within two weeks, you will likely appear clean!
The window period is not inherent in the disease. It is a technical limitation of the test. The test cannot find small quantities of infection. All measurements in science and engineering have limits, such as the smallest quantity (or change in quantity) that can be detected.
In addition, people can make mistakes or act irresponsibly. There can be fraud: a person with a fake ID taking the test on behalf of someone else.
Because of these problems, even if you insist on your partner being tested daily, you do not have an iron-clad assurance. And people don't have tests daily, because not only is that impractical, it is also based on complete fear and mistrust.
The relationship boils down to trust: your life is in the hands of the other person.
Never mind sex. For example, if one day my wife decides to use poisonous mushrooms to make 松茸ご飯, I do not stand a chance. I accept this, and just hope that in such a situation, death will be swift and painless.
I think that these two (トピ王と彼氏) should just go ahead and do it: *protected* sex. And also have the tests, too. People should do that if they are in a serious relationship.
See, this kind of paranoia is part of the reason why we have declining birth rates in the civilized world, especially among smart people. Meanwhile, dumb people are multiplying like bacteria, passing down their dumb genes to make more dumb people.