As per story plot, the age factor shouldn't matter, nor the factor of being reserved, atleast in Yumina's case.
Since unlike Touya who has the social norms of Japanese culture (which is why he suggested the 18 years clause to avoid being married), Yumina has the social norm of the other world, were girls her age are usually married and some even have kids already, since Yumina and most of the girls in Touya's Harem are already considered adult as per that world standards.
And if being reserved and them being supposedly underage is so important, than why does Yumina always sneaks into (or tries to) Touya's bed at night and sleep with him? That's not something a person who advise morality should do.
Also, being indecent and watching are two different things.
He was only watching his own fiancees/brides, not some random girl out there.
Plus MC is currently using illusion to look like a girl. And a girl looking at another girl isn't odd. It would be more odd if people notice a girl actively trying to avoid looking at another girl, especially if there is no social acceptance of homosexuality.
