Circumcision has nothing to do with AIDs, only HIV. (there is a major difference between the two)
The answer is, it doesn't. There is no proof circumcision decreases the chances of a male contracting HIV (or spreading for that matter). The only evidence SUGGESTING (not proving) so is correlational research which is not valid. Correlational research does not show imperical proof.
The best way the exsplain the theory however is just to say that the head of the penis and inner foreskin is mucus membrane, not normal skin. It's mucus membrane similar to that of the mout, inside the eye lids, and the vagina or rectum. Mucus membrane is thinner skin than normal skin and in the thinner skins there is a higher prevelence of a cell called Langerhan's cells. These cells are made from the bone marrow and are immune cells essentially and protect pathogens from infecting the body through the skin. Well since HIV directly targets the immune system they argued that HIV was more likely to be transmitted in an uncircumcised male because uncut males prosess more Langerhan's cells in the genital area. Because circumcision exsposes the head and inner foreskin, the skin undergoes a process called Kerantinization to protect the sensitive mucus membrane, thus the skin thickens and becomes more like your normal outer skin. This thickened covering covers most of those Langerhan's cells which is where the theory comes from. (the thickened skin is also what causes so much sensitivity loss in circumcised males) However when they do studies in labs, they have found HIV does NOT target or infect Langerhan's cells when entering the body, therefore the theory was disproven and is null and void.
In short, circumcision does nothing to curb any STD