Yuu & Marika's relationship reminds me of a real life couple with the same issues, Zhang Muyi & Miki Akama, where the girl was literally 12 y.o back then, & the guy was 24. You can google them if you want to know.
It's the same case with Yuu's here. Although it's a bit twisted to say that they both fell for a 12 yrs old girl, but I also don't think these both men would randomly attracted with any girl as long as they're underage, I just think that they both saw something special beyond physical which is happened that special thing to be inside a 12 yrs old girl. In Zhang's case, the relationship between him & Miki are still going strong, as far as I knew they both married in 2018, when Miki has already reached her legal age. They've been receiving mixed reactions from netizens, some of them are supportive, while the rest of course are giving negative reactions. And who am I to judge anyway? Sure for a normal society nowadays this case is considered a problem, but for some ppl it's considered as love, I mean with Zhang & Miki's case it's a consensual relationship and Zhang is still with her even if Miki's already a grown up woman now.
I don't know much about how society in Japan interpret the term of love. But I've seen many news in Japan about guys/women married their sex doll, AI robot, plushies, their pets, anime characters, etc because they 'love' them, and some of them considered it normal while we who are living in the other part of the world see this as pyschological problems. Also regarding the underage thing, I happen to live in country where underage girls in rural areas could get married and have kids and it's considered normal, by the time they reach 30s they would already have grandkids.
And about Yuu & Marika, I think they both are obsessed with each other to the point they would do something out of the norm. Everyone here are pointing out abt Yuu's 'pedophile' behaviour, but don't you think Marika is also a bit twisted herself, she's so obsessed with him even after she knew Yuu's masturbating abt her, she's still in 'love' with him. By the time she reaches 20, she's determined to 'follow' Yuu to wherever he goes and stake a claim on him, looks like she also has a 'stalker' behaviour in herself. If we're talking about normal, they both are abnormal anyway.
Pardon my mistakes, english isn't my 1st language.