Volume 1: Sensei wa Butchozura Shite Yatte Kuru

Volume 2: Dekiai Kareshi wa Butchozura Shite Yatte Kuru

Volume 3: Geki Ama Kareshi wa Butchozura Shite Yatte Kuru

From Renta!:

There's one man Yura despises at the day care his nephew attends: Mr. Kaji, the super cold and unfriendly day-care teacher. He's nice to the kids, but he doesn't even smile for the parents and guardians. To make things worse, Yura is traumatized from a past relationship, and Mr. Kaji sees right through him. But then, one day, this teacher Yura can't stand protects him when Yura's ex is being a complete jerk... by pretending to be his boyfriend...!! This is a spin-off of "Dirty Daddy Wolf in Sheep's Clothing" about a romance between an unfriendly day-care teacher and an office worker.

From Exiled Rebels Scans:

Volume 1:
Yura, who picks up his nephew on the way home from work, has a natural enemy: The blunt and unsociable preschool teacher Kaji. Despite his kindness for the children he teaches, Kaji hasn’t a smile to spare for the guardians who come pick them up. Overlooked by the gay Yura, whom has a heart breaking history with his past boyfriend. When Yura’s unrequited love silences him, calling him a “bastard homosexual” Kaji steps in to protect him, pretending to be his boyfriend?!

The love between an unsociable preschool teacher x salary man

Volume 2:
Unsociable preschool teacher Kaji and salaryman Yura have been living together for half a year, filled with happiness and Kaji’s unexpected smiles. But there’s been a pleasant miscalculation of Kaji’s monster strength when it comes to the bedroom. Yura makes the effort because “I want to satisfy him too!” but is unexpectedly rejected… Maybe he’s getting tired of having sex with Yura!? Being happy but sometimes going through a rough patch – a sweet, sweet love story!