5 years is not a big difference. He's not in his mid 20s or 30s. Hiyori is 5 years older than Zoro, it's nothing.
How brainwashed do you have to be to think this way?
It's legal in Japan and my country, get over it, Sanji was planning on marrying Pudding, I don't remember Luffy and Nami freaking out and asking to see her birth certificate.
You have different values because of the laws of your country, deal with it.