In Search of the Lost Future (Ushinawareta Mirai o Motomete) is an anime series (12 episodes + OVA) based on an adult visual novel. Several members of a high school astronomy club are joined by a mysterious girl who came out of nowhere. This girl hides her true purpose and strives to protect another girl, who is a childhood friend of and loves the protagonist.
This anime has a harem aspect but it is not the focus of the story. It has a little fanservice (more in the OVA), but no sex. Before the anime progresses to the main point, we see some club member activities, although generic I find them pleasant to watch, as compared to the first several episodes of Steins;Gate which I’m not too excited about.
When the truth and time travel is finally revealed, it is already near the end of the series, and is therefore resolved fairly quickly. I find it interesting that the characters seem to make the worst possible choices, making the goal impossible to reach. I think the ending is good and nearly sufficient, although some viewers are dissatisfied. Steins;Gate had far more time and complexity in its time travel though.
My only gripe about the series is that it never properly explains the origin of the black box, since it is a central part of its time travel. In the OVA it is revealed that the black box has existed a long time before the characters are born.