I finally got Katari to have full sex and bottom for the mc/captain on that Sithyryan beach, it helps to have your mc/captain set be a dominant/top in the settings.
When you get to the beach in a drunken state Katari will resist bottoming and protest. But he will do it if you insist and not like it.
It becomes obvious Katari is frustrated due to being abused and dominated all his life, he can only truly assert a degree of power during sex which is why he often becomes aggressive sexually and doesn't like bottoming, for Katari bottoming is a form of being subjugated again.