If You Walk Among the Lillies You'll Run Into a Pole -- Yuri mo Arukeba Bou ni Ataru

This is a great hentai manga from artist To! Hegemonikon, his thid tankoubon release, about two lesbians who learn something about sex with boys, too.

A boy accidentally witnesses a lesbian scene between the two girls sitting next to him in class. He accidentally lets slip that he saw them, and they drag him to the infirmary to interrogate him about how much he saw. When he confessed he'd seen them climax, they begged, "We'll tell you anything, just keep this a secret," and made him watch them perform lesbian sex right in front of him... Of course, all three couldn't just watch and it escalated into a threesome!!

This volume encompasses the whole story across four long volumes.