Since modern romance often ties "love" to physical desire, people tend to forget that in literature (considering this manhwa is an adaptation of a novel with the same title), first love can also be about pure admiration, appreciating someone's beauty, or even a meaningful moment that leaves a deep impression without any sexual desire at all. And that's exactly what we see here. It's still valid to call it first love. You just need to understand that love can take many different forms, so don't apply your own understanding of love to what Yuri meant when he called it his "first love," because that's obviously going to make you feel icky. If you still find it weird because Yuri was 25 while Xinlu was 12, that's exactly why POV matters. We get to see how Yuri himself describes those feelings, and based on what's actually shown in the story, they don't fit the medical criteria or legal definition of pedophilia either.
It's explained later in the novel. His first love was not the sexual kind. It's the same as falling in love with your elementary teacher who made you happy, your first friend or your first pet.
mr. gable openly admitting he's a pedophile（/TДT)/ falling in love with a kid at his big age of 25 ╥﹏╥ i guess it's a good thing that they only met that time because if not and he stayed with xinlu, he will be a certified pedo groomer goddamn（/TДT)/