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.