By interacting with the handsome adventurer in the forest, who brings you to the lake, and picking the right option.
What I do know is that you will need a sex interest stat at least high enough to actually have sex with the handsome adventurer.
What I don't know for sure is how many times you need to do it, or where your submissive / dominant stat needs to be sitting.
Whatever the requirement is, going to the lake with him will eventually be interrupted by a bandit. Staying quiet and going with the bandit will lead to a scene where you have anal sex with him. He gives you the butt plug afterward.