The protagonist's status as a virgin got found out by a senior colleague named Sakura at a company drinking party!
He gets teased by the drunken Sakura and even her junior Maiko, who was also sitting with them when he let it slip.

The teasing didn't actually bother him that much, but it did bother the sober Sakura. Several days later, he gets invited to dinner with Sakura and Maiko.

At Sakura's apartment, as soon as they started eating dinner, Maiko somehow got herself smashed and fell asleep. So it was just him and Sakura; feeling uncomfortable with the awkward mood, he finished his meal and got up to leave. Sakura grabs onto his sleeve and offers to take his virginity...

Sakura seems to be worried over something else after having formed a relationship.
Maiko says that Sakura actually has a timid character in contrast with her looks, and he gets told to "do something" about it...

(Source: DLsite)