This one's a very interesting read. It's about a country where everyone is matched with a lifetime partner when they're 3-4 years old. However, being partners is different from being married as you can have both a lover and a partner (but your partner is always your most important person). Furthermore, no one places priority on love and romance, so even if two partners have have sex or develop a family, their relationship is still more platonic than anything since it comes from the mutual understanding and acceptance that they will always be together until they die.
Shinkuu Yuusetsu