Young love is a helluva drug.
No, seriously. People do all kinds of mental gymnastics, give away their everything and knowingly ruin their own lives just to indulge in that pitter-pattery burn a little bit more, basically the same way those well-educated, formerly successful homeless heroin addicts do everything they can to fuel their own addiction even as they vocally acknowledge they're making themselves miserable in front of a camera.
It's unreasonable to expect things like perspective, gratitude and logic from a person who is under one of the most potent and long-lasting natural drugs produced by your own body. Just like a drug, your first few times are going to be a trip where you're fully not in control. It doesn't matter if she's almost 20, especially if she's as sheltered a person as Haru is, and from a household as backwards and conservative as hers on top of that. There are some very innocent 18-year-olds out there, just as there are some 14-year-olds with vastly more romantic or sexual experience than people twice their age, so bringing up numbers is pretty much a moot point.
And lastly, if you're an older person, it can be nice to see your juniors indulging themselves in whatever life may have to offer. Love-induced selfishness is likely to just seem charming on a person if it's someone you've already come to like, after all.