The virgin membrane is a sign of virginity, so idols who must be virgins may insert their dicks as long as they are short enough not to damage the virgin membrane, right? That's the idea.
処女膜は処女の証なので処女じゃないといけないアイドルも処女膜を傷つけないくらいの短小ちんぽなら挿入してもいいのでは？って発想

https://www.pixiv.net/en/artworks/73383232