AU where, at least in some places, it's common for those in the second half of their teens to be 'given' to an established couple for some period of time to learn about sex, relationships, how households are managed, etc, and it is expected for them to at least sleep with the couple and watch them have sex, if not participate.

Any couple/any of the younger characters - Yuuri/Victor and Yuri, or maybe younger Victor and still-together Yakov/Lilia, younger Yuuri and Yuuko/Takeshi in what was perhaps mostly meant to be a move to help them deal with newborn triplets... AU couples are okay, too!