How is HIV transmitted?

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Body fluids
How HIV enters the bloodstream
What makes transmission likely?

HIV is usually transmitted sexually, although it can also be transmitted through sharing needles or from mother to child. For HIV transmission to occur as a result of sex between men, the following needs to be the case:

One of the men must have HIV (the section Is he likely to be HIV-positive? gives more detailed information about this).
The sex must involve body fluids that contain sufficient quantities of HIV.
These body fluids must get into the bloodstream of the negative man (the section How risky is...? has details about the kinds of sex that facilitate HIV transmission).

Body fluids

For HIV transmission to occur, HIV-infected body fluids have to pass into the bloodstream of an uninfected person. While HIV can be found in many different body fluids of a person with HIV, only some body fluids contain a sufficient quantity of the virus to enable HIV infection to occur. These body fluids are:

Blood
Cum and pre-cum
Discharge from STIs (such as gonorrhoea)
Anal mucus

Anal mucus is a naturally occurring fluid that lines the arse. Its main function in the body is to lubricate your shit as it passes. Research indicates that anal mucus is the body fluid with the highest concentration of HIV.1

You cannot become infected with HIV through exposure to piss or saliva.

How HIV enters the bloodstream

There are two ways for HIV to get into the blood of an uninfected person:

Directly into the bloodstream through damaged skin, injecting equipment or invasive surgical procedures.
Through mucous membranes.

Mucous membranes are thin tissues which protect openings in the human body. There are many mucous membranes in or on a person. They occur in places like the mouth, inside the eyelids, in the stomach and along the digestive tract. HIV can travel through the surface of a mucous membrane, enter the tiny blood vessels inside and attach itself to the mucous membrane.

However, the mucous membranes that are most commonly involved in HIV transmission are in:

The arse
The foreskin and head of the penis
The urethra (the tube you piss through)
The mouth and throat

With the exception of the mouth and throat, none of the mucous membranes need to be damaged to provide an effective route for HIV transmission to occur. However, damage to the mucous membranes, like a sore or a cut, does make transmission more likely. Of the four mucous membranes that can allow HIV to enter the bloodstream, the mucous membrane in the arse provides the most effective route for HIV transmission. This is because the mucous membrane in the arse is designed to absorb liquids directly into the bloodstream. It is therefore extremely efficient at absorbing HIV. The mucous membrane on the head of the penis of circumcised men hardens. This provides some protection when it is exposed to HIV-infected fluids. However, it is still a possible route of HIV transmission.

What makes transmission likely?

Some types of sex between men make HIV transmission more likely. The situation that is most likely to result in HIV transmission is:

A man with HIV (whether or not he has been diagnosed) fucking an HIV-negative man without using a condom and cumming inside him.

This situation is the most likely to facilitate HIV infection because:

A positive man fucking a negative man without a condom means that the negative man is exposed to HIV.
A positive man's cum is a body fluid that will usually have a sufficient quantity of HIV in it to infect the negative man.
The mucous membrane in the negative man's arse provides one of the most effective and efficient routes for HIV to enter the bloodstream.2

More details about the risks of different sexual acts, both for HIV and for other STIs, can be found in the section How risky is...?.

References:

1 Zuckerman RA, Whittington WLH, Celum CL, Collis TK, Lucchetti AJ, Sanchez JL, Hughes JP, Sanchez JL, Coombs RW. Higher concentrations of HIV RNA in rectal mucosa secretions than in blood and seminal plasma, among men who have sex with men, independent of antiretroviral therapy. Journal of Infectious Diseases, 2004;189:156-161.

2 Royce RA, Sena A, Cates W Jr, Cohen MS. Sexual transmission of HIV. New England Journal of Medicine, 1997;336:1072-1078.