Bojack didn’t know that Todd was asexual yet. After everything bojack has done and Todd saw him do, it was just another sign that bojack did whatever he wanted regardless of consequences. And Todd at this time still liked Emily even if he didn’t want sex so it’s still betrayal.

Also Bojack did other things that led up to Todd lashing out.

“You can’t keep doing shitty things” is about the past, not just Emily.