@mahtan yeah, it's basically the Greek view. Delicate = good, huge =bad.
Japanese view: the thing is, multiple girls already showed to be
sexually interested in him, there's no moral issue on returning those feelings. It would be different if he showed signs of not being interested in sex, but it's the opposite - staring at Leah's chest, idealizing sex, so goes on.
And even if there was some sort of moral issue, frankly? Good characters are relatable. We shouldn't expect a bunch of Mary Sues and Marty Stus that stick to a moral code flawlessly. For comparison, check Dr. Stone - Senku is Japanese, he's 18, and yet he takes some sips of some wine he's brewing (because he needs ethanol for sci stuff); why isn't he sticking to "nononono! I'm not 21 yet!"?