Needless to say, they're not real people. And on top of that, they're chibi-forms rather than lanky underdeveloped physique a true "underage" form would possess.
So while you can throw terms like "underage" and "shota" around - you're taking it out of context of reality.
Beauty might be subjective but the aesthetics aren't hard to break down. Mind - Body - Spirit.
You're purposing an attraction to the effeminate, which isn't the same as femininity. Females can't be effeminate - that would be cis. There's novelty in effeminate, see? You you look a puppy and think it's cute - that's arousal. Arousal doesn't mean "sexual" it means "engaging". All humans are wired to engage with neonatal characteristics. That's the essence of kawaii. It can be cute in body. It can be cute in mind - naivety. It can be cute in spirit - such as a endearing relationship between student and master.
Novelty is also arousing. So the unusual and abnormal become curiosities worth investigating. How people choose interpret their reaction is different from person to person. And that in itself is intriguing and the subject of many novels. Straight male attracted to Trap. "Is this love? Is this gay?"
How about the comical precociousness of a child with an insatiable libido?
In reality, notions of sexual abuse and incest are troubling. But in fiction they are campy fantasies such as time-clones, and family orgies that are part of sayian culture - you don't know!
Lacking the capacity to differentiate fiction from reality is a different matter. There are a lot of crazy's out there. Pedophiles? No no, I'm talking about the witch hunters that see fictional entertainment as evidence for thought-crimes. They think wrestling matches and TV personalities are real. Video games reinforce violent behavior. I mean if they think cartoon porn is bad, what other delusions do they have? As if everyone who watches Crime Drama is a sadistic freak, and only homely incels can appreciate Romance.