This is a ROUGH CHOICE in this situation, and honestly, I kinda like it.
Hold on don't get your pitchforks out listen. Think about this as a setting for a different manga. Like a school romance. Mysterious girl, turns into a cat at night, MC is like "oh wow dang" and then the romance is forced because if the girl doesn't have her love reciprocated she's forced to live her life as a cat. Kind of a Little Mermaid thing, the ticking clock, the characters must confront their feelings before its too late... you get it.
Except here she didn't fall in love with a boy from her class. She fell in love with an older dude who likes cats. Everything is different now. There's a real element of tragedy here that wouldn't be there if this clock and barrier weren't there. I'm not gonna sit here and say its totally fine that they HAVE to have sex before they're adults or they turn into cats, that's unsettling and I don't like it. I suppose "do it" could mean something else entirely, we haven't heard what "it" is and they keep talking about catgirls choosing to be cats or people, which implies something that can be done with your own will rather than a thing people have to agree to.
But the broad idea of someone running out of time to be loved is a classic of the romance genre, and this setup could work with no wrinkles at all in a different setting. A more cliche'd one would have literally no issues involved. Because of this, that makes the spin on this one feel weighty and sad in a way that it otherwise wouldn't.
I don't know how long this manga is, but there's a whole layer of characterization to deal with still. "Am I worthy" already came up, but there's a big looming "is this really what she should be doing" feeling that is just sort of hanging there. Shouldn't she find someone else? Why me? You know, those. But here that has actual weight; MC's been put in a situation where he's responsible for how the rest of a kid's life works out because she has a crush on him. That's unfair to him and dangerous for her. Working through those feelings would be a pretty strong arc and great ground for characterization.