This was originally published in a hentai magazine that's why it's tagged hentai, not because it has graphic sex scenes. In Japan the hentai label is used not just for hardcore porn but also softcore porn like this one. Velvet Kiss is another example. If it bothers you call it hentai you can call it Borderline-H, ecchi or whatever but as long as it's published in a hentai magazine it's officially considered hentai.

I've read this, and I must say that the hentai tag is wrong. It probably comes off as a hentai plot because of the instant marriage thing, but it's not hentai. Sure this manga has sex in it, but real couples have sex all the time. Do you remember Itoshi no Kana? That had a lot of sex in it too, but that, like this, is ecchi. The author is exploring the idea of progressive romance developing after the characters are in a sexual relationship. He's stumbling but he is doing well at illustrating that point. This is Borderline-H ecchi, yes; but this is not hentai.