When you’re the bottom (receptive partner):
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.
When you’re the top (insertive partner):
Topping someone without a condom lowers the chance of HIV transmission more than bottoming without a condom, but it is still one of the main routes.
If you are HIV-negative, topping someone bareback can lead to HIV infection. This is because the anal mucus that lines the rectum can contain a very high concentration of HIV. The mucous membrane just inside the tip of the penis and the foreskin can absorb liquids, like anal mucus, directly into the bloodstream. It is also possible that blood, if present, can be responsible for transmission.
Other infections, such as chlamydia, gonorrhoea, herpes, syphilis, warts and hepatitis can be passed to the top through his urethra (the hole at the top of your penis). Condoms can prevent most of the infections that you can get from topping, although it’s worth remembering that some STIs can be transmitted even if you use condoms.
While condoms offer protection against HIV and most STIs, they cannot prevent them all. Even if you always use condoms for anal sex it is recommend that you get regular sexual health screens at a STI/GUM clinic and continue to test for HIV twice a year.
Condoms can break during anal sex and this could make it possible for HIV or other STIs to be transmitted. Condom breaks usually occur because condoms are used incorrectly or are used for long sessions without changing them. If you use condoms correctly with plenty of water-based lube, it will greatly reduce the chances of them breaking.
If you are having group sex, it’s also important to change condoms for each partner. This is because it’s theoretically possible that traces of HIV-infected anal mucus or blood could remain on a condom after a guy with HIV is topped. This is also true for other STIs, including hepatitis C.