Yes, your first paragraph explains Kate's default path in a full harem, but if the player locks the lesbian content, she will change to straight once she tries Mjolnir. I know that is not why things work in real life, but it would be punishing players who don't like lesbian action if done otherwise.
Even if you acknowledge that it's not how it works in real life, that doesn't make it any less fucked up to have in your game.
And before anyone compares this to having incest, rape, or slavery or whatever self generally deemed fucked up in a game, this is different. Incest in most incest games is acknowledged as taboo, and is generally consensual. Rape is still treated, typically, as being fucked up, yea you'll be given the option or depending on the game only be able to rape people, but it's not treated as a good thing. Slavery is much as I described rape in most games. Changing someones sexuality is completely different. Evolving it is one thing, someone who thought they were straight or gay learning they're actually bisexual for example, but "flipping" someone and having it shown as normal and good is horrific.
Having Kate be path specific isn't punishing anyone. It's choice and consequence. It's like in other games having girls you can only get on the path of if you're willing to steal them from their boyfriend of husband etc. is that punishing people who aren't into that? No. What if a girl is only willing to Dom? Is it a punishment to have her locked behind having MC willing to sub to her? Again no.
A punishment would be if you're forced into a "bad ending" because you don't have all the girls or don't have certain girls.
A punishment would be if by not allowing the only, to my memory, out non-straight girl, to join the harem that one of the other bisexual leaning girls leave the harem to date her exclusively.
Now, I'm not trying to attack you or bully you into changing how you're going to handle your characters, it's your game you can do what you want with it, but I had to say what I said. If for no one else than myself.
(Edited to fix spelling error for clarity)