Also known as: ふたりエッチ
A relatively softcore and semi-Educational manga series that explores various aspects of sexuality and relationships, mostly through stories involving a newlywed Couple, Onoda Makoto and Onoda Yura.
Its basic plot was teaching a previously virginal man how to please his wife, who wasn't particularly sensitive, through the advice of his experienced older brother.
If used as a reference inside another work, it's probably to say "this character doesn't understand how sex works" and/or "you lost me".