Sono Otoko, Numa ni Tsuki.

Once you fall for this guy, there's no turning back. At 25, Momoka, still a virgin, turns down her first boyfriend's advances and ends up getting dumped. To drown her sorrows, she hits the bottle... only to wake up next to a total hottie she doesn't recognize? And guess what? This gorgeous guy happens to be a younger university student with a major player reputation...?! (Source: Petit Comic, translated)