Mahoutsukai no Yome, Suijin no Hanayome Recommendations
In both these manga's the heroine is transported to an world very different from their own. Both are betrayed and sold/sacrificed to a powerful magic user and become the bride of said magic user. Both the leading ladies have vibrant red hair and a desire to protect others. As for the male leads, they both know very little about emotions and have to learn human behaviour and to be more human themselves.
In both manga, a young girl starts out as a misfit in society, and because of that she ends up in an unconventional relationship with an ancient, powerful, and awkward being. The female MC is called his "bride" but in both cases the characters do not start out having romantic feelings for one another, and the characters do not have any marital or romantic expectations of each other. That being said, a slow, awkward, and confused romance does begin to grow between the characters as they come to know one another better.
The aspect that seems most similar to me is how the male