How risky is getting fucked without a condom? 

Most gay men who have HIV caught it from getting fucked without a condom. As far as gay sex goes, getting fucked without a condom, and having your partner cum inside you, is the riskiest thing you can do [1]. This is because the lining of the arse can absorb liquids directly into your bloodstream. If there's HIV in his cum, and it goes up your arse, that will be absorbed too. Getting fucked without him cumming inside you is lower risk but, as there is HIV in pre-cum too, there is still a risk of HIV transmission.
In group sex it's theoretically possible to catch HIV from getting fucked even if your partner is HIV-negative, if he has fucked someone who is HIV-positive and then fucks you immediately afterwards. This is because there could be traces of HIV-infected anal mucus or blood on his cock.

Getting fucked is also high risk for most other STIs, including chlamydia, gonorrhoea, herpes, syphilis, warts, hepatitis B and for HIV-positive men it is thought that you can catch 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 cock. If you are infected with an STI in your arse, it will increase the chances of you being infected with 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 person doing the fucking is HIV-positive and has an undetectable viral load the risk of transmission will be greatly reduced. 


What if I fuck without a condom?

Fucking someone without a condom is less risky than getting fucked without a condom, but it is still one of the riskiest sexual practices that gay men do.1 If you are HIV-negative, fucking someone bareback is more likely to lead to infection than sucking cock. This is because the anal mucus that lines the arse (we all have it) 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. HIV experts used to think that infection from the receptive partner (bottom) to the insertive partner (top) was as a result of bleeding in the arse. Although it's possible that blood is responsible for transmission in some cases, we now think that anal mucus is the body fluid that enables the man doing the fucking to become infected.
Other infections in or around his arse, such as chlamydia, gonorrhoea, herpes, syphilis, warts and hepatitis B can be passed to the guy fucking through his urethra (the tube you piss through). Condoms can prevent most of the infections that you can get from fucking, although it's worth remembering that some STIs can be transmitted even if you use condoms.

If the guy getting fucked is HIV-positive and has an undetectable viral load the risk of transmission is reduced, however levels of virus in anal mucus may not match the level of virus in the blood (which is how viral load is usually measured). 


How risky are fucking and getting fucked with condoms?

While it is rare, condoms can break during fucking 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 gets fucked. This is also true for other STIs, including hepatitis C. While condoms offer protection against HIV and most STIs, they cannot prevent them all. Even if you always use condoms for fucking we recommend that you get regular sexual health screens at a GUM clinic and continue to test for HIV on an annual basis. 

CONDOMS AND LUBE
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1 British Association for Sexual Health and HIV (BASHH). UK Guideline for the use of post-exposure prophylaxis for HIV following sexual exposure. International Journal of STD and AIDS, 2006;17:81-92.