Futamata Ren'ai: Two Times the Trouble

Our protagonist, Koga Nao, wakes up one morning in high spirits. The birds are singing... and a girl is with him. The girl, Toiro Kirame, he met the previous night at a college drinking party. While he'd only meant to take care of her after she'd had too much to drink, confusion and a gap in her memory leads to her mistakenly assuming their relationship and demanding he take responsibility by going out with her. However, the situation grows complicated when Nao's longtime crush, Nobuta Yua, responds to his many confessions out of nowhere and declares that he's now her boyfriend.