Something for Blue: Watashi no Shiwase wa Kekkon?

Synopsis

It's Kohane's 29th birthday. She's been living with her younger boyfriend, whom she's been dating since her student days, for six years. She's excited, wondering if he might propose. But they haven't had sex in a long time. She heads home with a faint glimmer of hope. But instead of a proposal, he brings a ridiculous suggestion...!? "How about we enjoy being single for about two years before getting married?" Kohane is pressured into accepting two years of infidelity... (Source: DPN Books, translated)