A drifting glow stick balloon floats by the ship and catches the attention of Charlotte. When Charlotte walks by to investigate, she hears noises from within the ship and opens the cabin door to discover both Penny and BoJack in bed, about to start undressing each other.

You already mentioned it but you seem to have forgotten that after his initial rejection the mom caught them about to undress each other.

https://bojackhorseman.fandom.com/wiki/Penny_Carson