Since you insist on making it yuri... "a yuri friend appears and makes a bid with them, they lose and the penalty is having sex for an entire month, whilst they can't see their boyfriends. They comply, have more pleasure than ever but after a month they don't feel anything for each other, so they don't have sex ever again. Nevertheless, they break up with their boyfriends because they're no longer satisfied and the manga ends in a panel where they go out to find their perfect yuri partners". Well, and the ex-boyfriends just go out with each other.