HIV is spread through certain bodily fluids: blood, cum (semen), pre-cum (pre-seminal fluid), anal fluids, vaginal fluids, and breast milk. And since for a lot of us sex is generally all about exchanging body fluids, we can be creating a scenario where the chance of HIV transmission is possible. If you are worried about other body fluids such as spit, sweat, or piss, they do NOT transmit HIV!
Since the advent of pre-exposure prophylaxis aka PrEP (a once a day pill that prevents HIV infection) many of us now have the option to decide what kind of sex (bareback or with a condom) we want to have with our sexual partners. HIV-negative folks who take this pill everyday (adherence is very important) reduce their chances of acquiring HIV by 92%-99% (prepfacts.org)
One other important piece of information in regards to HIV transmission was recently confirmed. In 2017, Centers for Disease Control and Prevention (CDC) stated, “Across three different studies, including thousands of couples and many thousand acts of sex without a condom or pre-exposure prophylaxis (PrEP),” the statement continues, “no HIV transmissions to an HIV-negative partner were observed when the HIV-positive person was virally suppressed. This means that people who take ART (anti-retroviral therapy) daily as prescribed and achieve and maintain an undetectable viral load have effectively no risk of sexually transmitting the virus to an HIV-negative partner.” (Read more here.)
In other words, if a HIV positive person is undetectable they are unable to transmit HIV.