I think it's not easy to find a hentai manga with real plot. But I encountered some hentai (not only smut) before and the story (plot) inside makes me surprised. The story can be deep sometimes.
I guess I'm not the only one who is scared of gigantic, unreal breast. And I also hate harem type.

These are some manga and some authors I read and really liked them

Itoshi no Kana
TANAKA Yutaka

HARUMI Chihiro
Koi wo Suru no ga Shigoto desu

NANAMI Yasuna : this is a doujinshi author but I like this type of drawing.

Please introduce me some more manga. I thank you for every suggestions. ^_^

Should this be redirected to 18+?

Whatever, I'll just put a

I read much less hentai/smut than I would like, mostly sticking to eroge. You sound like you might like eroge if you're looking for plot as well as hentai.

At any rate, maybe something like Happy Negative Marriage, although the art doesn't seem to be your style. I have to confess that when I read hentai, I don't usually do it for the plot. Good plot is obviously a plus, but it's not a requirement, so I don't have many suggestions in terms of doujinshi/manga.

Maybe Nana to Kaoru and Nana to Kaoru Arashi
Perhaps Sundome as well.

Other than that, I really can't think of anything with a great plot. I must've read a hundred or so doujinshis that I really liked, but I can't really remember any of their names.
Doujinshis usually aren't long enough to have a very coherent plot, other than an entertaining or realistic way to lead into the h-scene.

Musa

[img]http://img803.imageshack.us/img803/6167/avvy.png[/img]
Rape is imminent.

I have read Nana to Kaoru but it's not my type. I will try Happy negative marriage. The introduce of Musa is also very interesting.

Thank you for your suggestions.

I'm also a big lover of plot-oriented H, especially when the girl's drawn with normal breasts. I tend to prefer small, just because I've seen many authors have a lot of trouble with "average", so that the girls can gain and lose multiple cup sizes every few pages; the flatter the girl is, the easier it is for them. One author I'd recommend is Hanamizawa Q-tarou. Lots of plot in some of his stuff. Unfortunately, currently only REC and Momoiro Sango have partial translations; gems like Honey Blue and Tsuukai Suzuran Doori are only available raw.

Also good are Koe de Oshigoto and Momoiro Peanuts.