Let’s set a general rule, shall we? Whenever there is body fluid involved – be it vaginal fluids or semen – the risk of STIs, STDs, or AIDS exists. Eating or swallowing cum involves body fluid, ergo, the risk exists.
Now ready for a surprise? When HIV is concerned, there’s little, to no risk when swallowing cum is a comparatively less risky safer option!
But transmission of HIV, though extremely rare, is theoretically possible if an HIV-positive man ejaculates in his partner’s mouth during oral sex. Weren’t expecting that, were you? Well, that’s because the entry point for HIV is much more likely to be via a cut in your gums than the stomach. More importantly, the risk stays low as long as you spit or swallow quickly rather than keep the cum in your mouth.
But that’s just HIV. What about STIs like chlamydia, gonorrhea, and syphilis? The risk from oral sex exists as the cum still passes through the mouth but given that these are all bacterial infections, they can be easily treated and cured.