HIV antiretroviral therapy (ART) reduces the chance of transmitting HIV. With an undetectable viral load, in many cases this risk is close to zero. Although this has been known for many years, there is now much better evidence, including for gay men as well as heterosexual couples.
The PARTNER study reported no transmissions from the positive partner with an undetectable viral load. This was after 44,500 times that people had vaginal or anal sex without condoms. This study is still ongoing for gay men because there were more heterosexual couples in the first part of the study.