It doesn't though. You don't even have to start Bisette as far as I'm aware. As far as I remember it all you need for Roxxy are Jenny's panties to get some Dex and that's only because her story isn't reworked yet. Nothing else is needed.
I'd agree that may change later once the game is done but right now there's really no connection.
To just play the Roxxy stuff, if you're not cheating, you have to:
- Increase CHR by doing rap battle mini-games to get past the locker room shower scene where Becca and Missy are blocking you;
- Increase STR to succeed in beating Dexter in the second fight, which means progressing Erik's path to the point that he's taking over cafeteria duty and Kevin is freed up to train you;
- Increase DEX through Master Somrak training, which means progressing Sister path through at least two rounds of buying panties off of her after trading sex toys;
- You have to do at least one of the homework assignments for Miss Bissette (I think it's only after the second one that you get the prompt to tutor Roxxy, but that just might have been how it was in my playthrough)