You may be the only one bleeding, but you can have cuts anywhere on the body and if blood is flying around (which is can) some could go in the mouth, eye, up the nostril etc, so it is still a risk to one party.
I imagine it can be quite hard work creating those cuts too and hands can get sores etc and breaks in skin. I have broken a nail spanking before now and as I said before, I'm a softie.
I'm not saying that anyone will get HIV, but just trying to address the balance and show that there is a risk (however small) in both activities.