Ringoku kara Kita Yome ga Kawaisugite Doushiyou.

Saryu, known as the Winter Bear of Tidros for his formidable presence that scatters enemies, suddenly finds himself in an engagement with Lady Sitoen, a noblewoman from a neighboring kingdom whose previous engagement was called off! Starting with a forehead kiss, he's nervous around a woman he's not used to. Her smile makes his heart race, and a glimpse of her chest leaves him flustered. As improper fantasies run wild in his mind, Saryu's rationality is on the verge of collapse?!

(Source: Shufu to Seikatsu Sha, translated)