Many HIV-negative men think it's safe for them to fuck without condoms, even with a partner who is, or might be, HIV-positive, as long as they are the top (i.e. the guy doing the fucking). Similarly, many HIV-positive men think they can fuck without condoms without putting an HIV-negative partner at risk, so long as the HIV-positive guy is the one being fucked.
Many men believe this because they have heard that it is easier for HIV to travel from the guy who is fucking to the guy being fucked than the other way round. Although it is easier for HIV to pass from the top to the bottom, many men become HIV-positive as the top (i.e. insertive partner) when fucking without a condom.
No. Choosing to be the insertive partner is NOT a reliable technique for avoiding getting HIV during condomless anal sex. Many men have become HIV-positive as a result of being the top during unprotected sex.
HIV in body fluids in the bottom's anus can enter the top's cock through the urethra (i.e. the opening at the tip of the penis through which urine and cum passes). It can also enter the cock through any cuts or abrasions caused by friction during sex, or through sores or other irritations caused by the presence of another STI.