I am a bisexual man and 100 TOP when with another man. I have been told that the risk of HIV infection for a TOP during unprotected anal sex is small. I would like your opinion.
The risk is small compared to what? Risk for HIV infection is always relative, so make sure that you understand the context when getting information about transmission.
Terms like "high" or "low" risk are often necessary, but not always useful. For instance, compared with being the bottom in anal sex, being the top is often considered low risk. However, compared to unprotected oral sex without ejaculation, being the top in unprotected anal sex could be considered high risk. There are multiple variables that are considered in each encounter that change the relative risk as well (how much fluid were you exposed to, how much virus were you exposed to, what was your immmune response, etc.).
Risk is also based on a person's subjective interpretation of risk. One person might consider performing oral sex on an HIV infected man high risk, while another might consider it low risk.
So, high or low risk might not be useful in the situation you describe. The facts might be more relevant:
A person who is the receptive partner in anal sex is considered to be at a greater risk for infection than the insertive partner. The insertive partner is not without risk if no condom is used.
Exposure to HIV infected blood poses a significant risk for infection. Having anal sex can cause rectal bleeding, which increases the risk for the insertive partner.
The odds of transmission in one episode of UNPROTECTED anal sex with an infected partner are lower than many people think. However, the more unprotected anal sex you have, the greater your risk for exposure becomes over time. Using a condom greatly reduces the odds of infection.
Men who have sex with men are at increased risk for exposure to HIV in the U.S. The more male partners you have, the greater the odds are that your partner will be HIV infected.