When we’re talking about anal sex, being topped by a guy without a condom, and having your partner cum inside you, is the most likely route for HIV. This is because the lining of your rectum can absorb liquids directly into your bloodstream. If there’s HIV in his cum and it goes into your rectum that will be absorbed. If you are topped without a condom and he doesn’t cum inside you, there is a lesser chance but, as there is HIV in pre-cum too, HIV transmission can still happen.
Bottoming without condoms is also a main route for most other STIs, including chlamydia, gonorrhoea, herpes, syphilis, LGV, warts, hepatitis A, B and increasingly hepatitis C as well. Condoms provide an effective barrier against most STIs, including HIV, although some STIs, such as syphilis and warts, can still be transmitted if the condom does not cover the entire infected area, such as the base of the penis.
If you have an STI in your rectum, it will increase the chances of you getting HIV if you are HIV-negative. If you are HIV-positive and have an STI, it is likely that there will be higher concentrations of HIV in all of your body fluids, including blood and anal mucus, and so you will be more infectious. If the top is HIV-positive and has an undetectable viral load the risk of transmission is zero, once he has no other STIs. Learn more about being undetectable.