well it looks like you have two options.
1. Have agility stat at 8 and increase arousal until you can masturbate at camp.
2. Have both companions Trudy and Kylara and again raise arousal until you can masturbate
I don't know if you need a charisma check for the friends, but in the reblog it listed an enemy putting you into the position, I haven't figured out who, but I have suspicions that it's the brigand, but I haven't really gotten much of a chance to test it out.