It's embarrassing to see Kotaru fall for this obvious trap.
He could easily have answered the jock bastard that there is an obvious difference between them. In Kotaru's case, he has discovered that there are two persons he is interested in, but this is recent, so he hasn't figured out what to do about it. Also, Kotaru is serious.
in the jock bastard's case, juggling a harem without any thought for the girls' feelings, or intention to stay with the "love interest", or seriousness, for that matter - all of these are par for the course, and standard operational procedure.
If the jock doesn't see or understand the difference, well, that's his problem, but Kotaru should tell Kuro this, so she understands what she's doing.
I guess Kuro's actions can be ascribed to innocence, and being new to a human body, and the feelings she has been discovering. But she should understand what her actions will lead to, both for her and for Kotaru (and potentially, for the curse).
Imagine how fun it would be for her if she loses his love, and he also doesn't have any children, so she's locked in the curse cat state.
Yay? Or even Double-yay?