I dunno the way I think about it is it’s a breach of trust in a friendship, and as others have said the “last straw” for Todd. The act itself isn’t that bad, they are two consenting adults who had sex with each.

But this was Todd’s closest and longest standing friend. Bojack could have sex with literally anybody else, but chooses to do it with someone that Todd clearly has close emotional ties too. Then it’s the fact that they lied after-like if it wasn’t a shitty thing to do that they knew would hurt Todd, why lie/cover it up? Also to add on to it Emily’s guilt made their relationship strain and she sort of exited Todd’s life (not permanently) at the moment, and he was super stoked to have that relationship again. While that was Emily’s choice, Bojacks actions played definite role.

And as far as the last straw goes, Todd had been processing all the shitty things Bojack had done to him since he found out about the rock opera, and while this under other circumstance may not have ended this this was it for Todd. And in my opinion justly so.