This is one truly amazing manga, but the most surprising thing about it, is probably the fact that it was written by a H author.
If I were to summarize what this series was about, I would say it was about love and not just about romantic love, but about love in general.
The story takes place on apocalyptic\post-apocalyptic earth where humanity is dying and those still alive suffer. The author is rather cryptic as to how and why all that is happening, but I think it had something to do with humans reaching the last stage of evolution. It's all rather confusing, but at the end of the day it's not what is important in this manga.
At the beginning of the series we learn that the male lead is terminally ill. As his death is approaching he decides to spend his few remaining months with an artificially created girl (it was never fully explained what she was, so don't quote me on this) who are offered to people like him for comfort. However the life span of those artificially created humans isn't long either. Allow me to quote "It's too bad she won't live!"*
Despite those difficulties (and through them as well) the two main characters come to truly love each other as they spend their remaining days together. However this series goes beyond the love between a man and a woman. When they come to terms with their cruel fate, they decide to share their love with others. Also other characters show up to widen the subject. Even the romantic love is more about cherishing** the other person than sexuality. Well, it's not exactly void of erotism, but make no mistake, it's not another Itoshi no Kana (despite sharing the author and a number of themes).
On a number of occasions the characters in the series have those depressing monologues in which they state that anything of value like religion, love and even science has no meaning to them, yet when facing things that challenge their humanity, they can prove that it's not true.
The only thing left for me to say would be to recommend you reading it yourself. It's a good manga.
* It's a famous line from Blade Runner. It's followed by "But then again, who does?"
** English isn't my native language, so I hope that what I wrote there is not complete nonsense.