While HIV is certainly the big dog, there are other STDs out there such as syphilis, herpes, gonorrhea, chlamydia, and genital warts. Protecting yourself and your sex partner is still important. Unfortunately, once you are infected with HIV it can be easier to be infected by other STDs, and the infections are often much more serious, like with syphilis.
Some HIV-positive individuals "serosort", meaning they only have sex with other HIV-positive people. This helps reduce the risk of spreading HIV to non-infected members of the community, but does not protect an HIV-positive individual from other STDs.
The other issue is re-infection - being infected with a different strain of HIV. Even if someone knew who they got HIV from, each body is it's own incubator for HIV and can produce very different types of virus over time. There are many different strains of HIV, some more resistant to HIV medications than others. We don't know everything about re-infection. But it is important to recognize that medication available to you now may not work in the future if you get a different strain.
Using condoms every time is a challenge for everyone, HIV-positive or HIV-negative. It can be especially challenging if your HIV meds make it difficult to maintain a hardon. If the combination of your HIV meds and a condom is making erections difficult, consider using a cock ring. Or consider using a bottom condom instead of a traditional male condom.
Ultimately, using condoms is important to prevent the spread of HIV. "HIV stops with me."