You missed some things. First, "If you learn sexual skills through her" the very important use of the word "If". Second, I'm not the dev. I did not create her or her role. Her role is "Master" the shorten description of this role is "Someone you learn skills, talents and who provides quest".
IF the dev created her with the purpose of learning sexual skills, my answer to 1) is the most logical way to go through it. IF it does not, then my answer has no merit. It's not a question of taste, what I like or dislike, just a viewing of what is currently in-game and making an answer based on speculation. I even said I would 100% agree if it wasn't for the ROLE of the cousin, how you managed to see this as my taste is baffling.
Third, "I don't like male character use other female characters..." the cousin was 100% in control during her scene. IF it happens the way I'm thinking it will, she would be the one using him and he just learning something in the process. Again, I'm talking less on taste and more on speculation using what is seen in game so far.