A question that gets asked a lot on the Poz.com forum. I will give the standard answer here that they give over there. Absolutely no HIV risk whatsoever. The protein coating on the virus deteriorates upon contact with the air, sweat, spit and and anything else., so even if the virus RNA itself is intact it cannot enter your cells and replicate so it just dies. Unless her cock went inside your arse or your cock went in hers, you had no exposure. It is very hard to transmit HIV, other STI's are a different subject. Much easier to transmit. Just get yourself screened for STI's as per any sensible schedule, but I would not worry about HIV from this encounter.