Koibito to Soto de H Shitara Shinsetsu na Yankee ni NTR Sareta Hanashi

Sayaka and Yuuki are childhood friends who have finally become lovers. One day, while they’re having sex in a public restroom, a delinquent from the academy they go to barges in and tells Yuuki he’s doing things all wrong. The bad boy, with completely good intentions, then goes on to show them both how you fuck the right way.

(Source: Irodori Comics)