Ok, I'm sorry for being unclear. When I'm dry humping hard in that situation, what are the chances of blood or something else leaking through the two layers of underwear and getting some kind of exposure to the urethra or head of penis?
Not to be harsh but might want to think about what your doing and know what your doing before you do anything else sexually.
Now your taking about vaginal fluids when you had "anal sex". Not sure what exactly your looking for based on this post and follow up question.
What are the chances of being exposed to vaginal fluid through the cloth after or even before the climax?
How is it possible to have anal sex while both people are wearing underwear? It's impossible to penitrate her.