How risky is sucking cock?
It is possible to get HIV from sucking cock but the likelihood of this occurring is low. Very few whose only reported risk is from oral sex have caught HIV, which leads us to believe that the risk is relatively low but also indicates that there is some risk. Oral sex is the most frequent kind of sex for men, so it says a lot that so few men have caught HIV from oral sex. We do know that well over 95% of HIV infections in gay men are due to fucking.

The reason that oral sex is so much safer than getting fucked is that the throat is not as vulnerable to infection as the arse is. Saliva has properties that can disable some infections, including HIV, and so there needs to be quite a lot of HIV present for infection to take place. Also, the mouth and throat are not as efficient at absorbing liquids into the bloodstream as the lining of the arse is. Even if you swallow cum, any HIV that may be there will usually be killed by the strong acids in your stomach.

How do I reduce the risk?
Although the HIV risk from sucking cock is relatively low, there are ways that you can reduce the risk further. It's extremely unlikely that you could catch HIV from giving someone a blowjob if they don't cum in your mouth. Although there is HIV in the pre-cum of a man with HIV, there is less than there is in his cum. This is because a man will generally produce more cum than he does pre-cum, and because cum has a higher concentration of HIV than pre-cum. The protective properties in saliva would usually be able to disable the amount of HIV that there is in pre-cum. A condom, used correctly, will prevent either cum or pre-cum getting in the mouth, although few gay men in the UK use condoms for oral sex.

You will be more vulnerable to infections if you have bleeding, scratched or damaged gums, mouth ulcers or a sore throat. People with gum disease or ulcers will be more likely to catch HIV or other infections from oral sex. It may be wise to avoid brushing your teeth just before giving someone a blowjob, particularly if you want him to cum in your mouth. Using a mouthwash before or immediately after oral sex will remove the protective properties that saliva contains and so may increase the risk of transmission.

If the guy getting sucked off is HIV-positive, on treatment and has an undetectable viral load then there is no risk of transmission. 

Can I get other STIs from sucking cock?
Although the risk of HIV infection is fairly low, some other STIs can be easily passed via oral sex, such as chlamydia, gonorrhoea, herpes or syphilis. These risks can be avoided by using a condom for oral sex, although many men prefer to take the risk than taste the rubber. There is a small risk of picking up hepatitis B through oral sex, if you have not been vaccinated. If you have an existing STI in your throat you will be more vulnerable to infection and the chances of picking up HIV or Hep B are increased.

How risky is getting sucked off?
No one has ever caught HIV from having their cock sucked. However, you may be vulnerable to other STIs this way, such as chlamydia, gonorrhoea, herpes and syphilis. This is because all of these STIs are much more infectious than HIV. If you wanted to prevent the small risk of getting an STI you should wear a condom for oral sex. As most men choose not to do this, it's advisable to have a regular sexual health check-up to ensure that any infections you may pick up can be detected and treated quickly.