Tadashikunai Renai no Susume

From Japanzai:
Ichihara Tatsujou, a 21-year-old virgin, begins dating Asahina Riko, a beautiful woman who is inexplicably interested in him. After the two begin dating, however, she reveals that she’s only interested in virgins; as soon as the two have sex, she will dump him. Unwilling to give up on his first-ever girlfriend, Tatsujou makes it his mission to convince Riko that there’s more to love than a short fling, ending after sex. At the same time, Riko concludes that she will seduce Tatsujou by any means necessary, and then immediately break up with him.