19-sai no Otto ni wa Himitsu ga Aru

An office lady in her thirties who works in the secretarial department, Aina Tanaka, is engaged to a rich heir and was supposed to have a smooth sailing life. However her fiancé suddenly dumped her for a junior colleague. Trying get over the breakup, she got drunk and collapsed on the street. Then she was saved by a young man... and they ended up spending the night together!

She was worried about having committed a crime, but the young man, after having heard of her plight, said he's about to turn 19 and offered to marry her under an arranged contract. Aina wanted to let her ex-fiancee think that she'd moved on, so she agreed...

But on their wedding night, it turned out that the young man was an unexperienced virgin...?

(Source: Amucomi, translated)