If Champ's horni and has already demonstrated a taste for this particular mino's dick, automatically fellating him doesn't seem like a particularly severe "punishment".
The way the initial encounter is phrased, it's clearly a "He's really on-edge because the entire place is swarming with imps. Maybe a kind BJ would calm him." not "I have a massive craving for minotaur cock down my throat at all times. If I so much as SEE a minotaur, and my face isn't at crotch level, my day is ruined." situation.
I wanted it to be a one-time thing, because a lot of encounters in this game are case-by-case basis. It would be like agreeing to do Elf sex with Hashat once and from then on without warning Hashat has a chance to show up and instantly and unconsentually rail you on the spot.
Admittedly you don't really NEED to use Livernus's boat at the moment, but considering how annoying the Undermountain enemies can get right now, going the long way might result in several losses depending on the situation.