During the camp encounter where Selkie sneaks into Hiro's tent the game increases your Blowjob Expert rank by one no matter how it plays out. There's probably a redundant rank increase prompt at the beginning of the encounter.
After doing some more testing, this turned out to be an odder issue than I thought. This only happens when you have 'smart AI' toggled. I usually turn that on when fighting goblins because they're less likely to get sticky fingers (well, at least when it comes to my potions), and for some reason this is what causes the rogue rank-up.