Adding a choice itself is not most of the work. That’s like ten minutes.
Most of the work would be in rewriting the sex scenes and stories to accommodate going either way, which gets especially hard when the characters themselves have moderately strong preferences. Just with Chester as an example, there are several days and a whole event with Darius dealing with how the MC could possibly deal with the bull’s endowment. With Darius’s route, topping and bottoming is tied up tightly in the progression of his character arc and writing it to go either way might not work out particularly well.