[enflove]

From Tsugumomo volume 6
you can download the manga there:
hXXp://www.raw1st.org/2011/08/tsugumomo.html

from what I understood, the girl is from a lineage of female fighters who have to marry whoever defeats them.
The hero beat her in a previous volume, and apparently now her (scary) mother wants them to marry each other; so they pretend to be lover and are forced to share a bath, and later a bed together.