Tsunagaranai Keitai Denwa -Tadaima Hoka no Otoko to Mechamecha Sex Chuu-

The two of them were happy. Not a single doubt clouded their days. At least, until they decided to get engaged. The protagonist, Yuu Takahashi, had only one lingering worry: his fiancée Nanako Ukai’s complete inability to hold her liquor. She loved to drink despite being a lightweight. If that were all, it wouldn’t have been an issue, but whenever she got drunk, she would completely black out, losing all memory of what happened. Of course, both of them were well aware of this and took every precaution, and their friends from college always kept a watchful eye on her at parties to make sure no “mistakes” were ever made.