Yeah so when I read the title and synopsis I assumed this was like sensei lock on, where the MC was a full grown working adult and had discussed things with their parents who supported them. Just saying, I don't get comments going against the female lead for seeing this as something immoral or wrong... because it is. On top of being gross, the reason why relationships with such massive age gaps with minors is frowned upon is that no matter what you're taking advantage of them in some way. Brain development doesn't finish until closer to 25 and even late teenagers are a mess of emotions and completely lacking in experiences. This is someone whose been crushing on their teacher since childhood.
That said trying to get married is about the dumbest way to go about this. I don't even know how that's a dating app. Going on casual dates with others does make sense then. I'd say the 3 year wait also makes sense. He should date other woman in that time or be open to the idea, but it's not like he has to. The point is giving him time and space so that a childish crush isn't controlling his decision making so much. Like at least 3 years later it'd be harder to say it's just a crush from childhood.
Really just saying this because of all the comments (that I find super sus) chatizing the Female lead for trying to get the Male lead to move past her. That does make sense and is adult. Beyond what society thinks too. Even an 18 year old will just lack the experiences and development of older adults and it very much feels like taking advantage of them. There are a lot of reasons people would raise their eyebrows at "how did you two meet?" "Oh I was his teacher when he was a literal child." and honestly one very real one is this very situation. He's held onto this crush for so long and has had so little actual life experience and hasn't even considered any other women. It's questionable because even if he's a legal adult (also to note adult age in Japan was only relatively recently moved down to 18 (US as well actually) and it's still traditionally 20), it doesn't feel like a relationship that started on equal footing. It feels like the boy was imprinted on as a child, held onto the obsession through development, and as soon as he turned 18 acted on it first thing.
Fictionally it's just a story and entertaining drama. I'm only really calling it out because some of the comments felt a little too serious. Like there's one Manga I like where a subplot is a teacher teasing a student who obviously likes her and they get married and start dating once he's an adult. Cute in Manga, but irl that's called grooming.