Well, if there was someone already hiding in the anus of someone undergoeing Anal Plundering, that would mean that one of two things happens
1: The spelunker went in head first, and he might end up raped as well.
2: the spelunker went in feet first, or pulled a u-turn, and his head is now oriented towards the opening. This means that if he brought no weapons, he will be given oral, but has the option to use a bite attack, or if he brought weapons, he can try and internally fend off the Anal Plundering attempt. This makes me wonder, what kind of HD would a penis have to resist destruction? Also, what AC would it have? I mean, it is unarmored, (assuming no condoms) so no bonuses there, but it would, on the other hand, get a full Dexterity bonus to its AC, right?
This is important shit, we need to figure it out.
Also, what check would be necessary to pull a u-turn in the anus of someone you are currently spelunking?