Before you go to the beach with Katari you must change your MC sexual preferences to TOP /DOMINANT, then after you have your drinks at the seedy bar and go to the beach Katari will reluctantly accept being made to bottom sexually.
Katari often becomes aggressive sexually and this is the way he vents his frustrations and anger due to the fact that he has to been abused and repressed all his life by various people. During sex he can finally achieve a degree of control by being sexually dominant and this also explains why he doesn't like bottoming. Bottoming for Katari means that he is being submissive or subjugated yet again.
Katari's past history of being abused and subjugated also explains why he thoroughly enjoyed raping Rylan In Galliano's Bar in front of the entire crew. Katari felt that Rylan needed to be humiliated, because Rylan had a low opinion of Katari and often insulted him to his face.
Another very important reason why Katari raped Rylan publicly was that Katari figured out that it was Rylan who disabled the spaceships cloak, shield and defences and this resulted in the spaceship being invaded and its crew being killed when Bradley invaded. Katari wanted Rylan to be punished for that too, at least to some degree. Because Rylan was not being tried in a court of law and facing a prison sentence for his actions.