Well, IIRC(feel free to correct me, just trying to remember that) the immune system consists of 3 types of cells, which I will call A, B and C now since I do not remember the official names. The A cells are patrolling the body and destroy every strange thing they can find that should not be there, they are the first defense. The B type will search for similar things and especially for infected cells, however it will not do much itself except to "activate" the C type cells which are able to identify and destroy infected cells with the information they get from the B cells.
The HIV virus now infects the B cells and reproduces itself inside them, destroying them(that's what a virus does, only that other virii use other cells as hosts), this will basically render 2/3 of your immune system useless because the B cells are destroyed by the virus and the C cells will never be activated again by them. This leaves you with the A cells but those are no good in defeating a mass invasion of virii or bacteria and cannot do much against infected body cells. I think a flu for example can then end up lethal and kill you.
Now circumcision can only prevent fluids from staying around underneath the skin of your penis and get sucked up by your skin I think, maybe cleaning the penis with alcohol shortly after sex might work as well.