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Scat and shit
The main risks with shit are not to do with HIV, but the transmission of a large range of other organisms (including viruses, bacteria, and parasites). These can be especially severe in someone who is seriously immuno-compromised. In principle, shit on healthy unbroken skin would carry no risk as long as you washed very carefully afterwards (including under the fingernails) and made absolutely sure not to get any in your mouth. However, the danger is that you cannot be absolutely sure of cleaning away all the highly infectious micro-organisms involved.
But if you are turned on by the fantasy there are ways of reducing the risk:
You could try alternatives like half–chewed Christmas cake for their shock value! Obviously there is no risk here. Or you might get a similar symbolic pleasure from other `mucky' activities like mud wrestling.
Some people can get the symbolic pleasure of a bum sitting on their face without the risk by using a barrier such as clingfilm or latex, but be careful not to smother yourself: you cannot breathe through clingfilm.
It is less risky to play with shit on clothes which you can then wash in the hot cycle of a washing machine, and it is less risky if you use rubber gloves.
And obviously if you do decide to play with shit then you can reduce the risks by not getting any in your body (mouth, vagina, eyes, cuts etc.), and by washing everything as thoroughly as possible.