Having unprotected penetrative sex is the easiest way to pass on a sexually transmitted infection (STI).
Using a condom helps protect against HIV and cuts the risk of getting many other STIs.
There are more gay men living with HIV than ever, so having sex without using a condom is extremely risky.
Gary Williams from Birmingham's Healthy Gay Life project says that many STIs are more difficult to treat if you've got HIV. Some, like syphilis, may even accelerate HIV's progression.
"We're also seeing a rise in cases of hepatitis C, particularly in men who have HIV. Hepatitis C is treatable in some cases, but it's a long and drawn-out process. So to prevent its spread, use a condom."
Screening for hepatitis C isn't routinely carried out, but if you think you're at risk or have been exposed, speak to your GP.
Find your local hepatitis C support service.
"Gay men should have a check-up at least every six months at a sexual health clinic because, for some infections, you will not see any symptoms," says Williams.
Gonorrhoea ('the clap')
This bacterial infection can cause stinging when urinating or the feeling that you want to urinate but can't. It's treated with antibiotics.
Read more about gonorrhoea.
Non-specific urethritis (NSU)
This is an inflammation of the urethra that's caused by bacteria. It's caught in the same way as gonorrhoea and often has similar symptoms.
NSU can also be caused by having lots of sex or masturbating a lot, which causes the urethra to become inflamed. It can be treated using antibiotics.
Read more about NSU.
This is a bacterial infection of the urethra, rectum or throat. There may be a discharge and pain when passing urine or pain in the testicles (although chlamydia can be symptom-free).
It can be caught during sex with an infected person in the same way as gonorrhoea and NSU. It's treated with antibiotics.
Read more about chlamydia.
Genital herpes is a viral infection. Symptoms can include painful blisters and ulcers on or around the penis or anus, although some men have no symptoms.
Genital herpes can be caught through oral sex with someone with a cold sore around or in their mouth, or by close, skin-to-skin genital contact with someone who has genital herpes.
Antiviral tablets can help the healing process and shorten the length of the episode. A GP can prescribe tablets or cream.
Read more about genital herpes.
Syphilis is a bacterial infection that causes a painless ulcer, usually in the genital area. It will disappear on its own but other symptoms may appear, such as a rash on the body and swollen glands.
In its early stages, syphilis is very infectious and can be passed on by close skin contact during sex. Treatment is with antibiotic injections or tablets.
Read more about syphilis.
This is a common infection that appears a few weeks or months after sex with an infected person. It can cause pinhead-sized growths, mostly on or around the head of the penis but also in and around the anus.
The sooner warts are treated, the easier they are to deal with. You can't treat genital warts with the same cream you use for warts on the hands. A doctor will freeze them or use a cream to remove them.
Read more about genital warts.
Pubic lice ('crabs')
Crabs are the most common STI and live in body hair. They only grow to pinhead size so can be difficult to spot, although their tiny dark eggs can be seen stuck to hair.
Crabs prefer pubic hair (hair around your testicles and anus) but can also be found in body hair (but not head hair). The lice can be picked up from clothes, towels and bedding, and symptoms include itching or a rash.
Treatment can be done at home with lotions bought at a chemist (no prescription is needed).
Read more about crabs.
This is an infection caused by invisible mites that burrow under the skin. It causes intense itching for most people (though some hardly notice it).
Itching usually starts two or more weeks after sex with an infected person. You can get scabies from sharing beds and towels, but this is less common.
Treatment is similar to treating crabs, although you may continue to itch for a few weeks after the mites have died.
Read more about scabies.
If you have any of the symptoms above or are worried you may have an STI, speak to your GP or visit a GUM clinic. Getting tested regularly is a good idea to ensure you have a healthy sex life. NHS services are free.
Find your local sexual health service.