The importance of sex

In the novel the author so far has displayed that the two characters Niska and Xavier have sexual desires. I feel as though putting those scenes in the novel were not exactly necessary to include but shows that the characters are human as well. After reading the chapter Seducing (pages 130-135) which was  told in Niskas point of view it honestly was not what I was expecting. Reason being is that in the beginning Niska was trying to avoid the white settled town at all costs and was very reluctant to be among them, which gave me the impression she despised or was scared and un knowing of them. In the chapter she quite literally hunted the white hunter and seduced him and then the reader gets a very explicit idea of what goes down between the two. At the time though I had not known or had any reason to think Niska is bitter toward the white people as she had an enjoyable experience with the hunter. After reading the chapter Betrayal (pages 164-176) it made sense to me as to why Niska has those feeling towards the white people.  I can see how in the event of getting betrayed,tricked and called a "squaw whore" by the man you identified as your mate would make a person bitter. In Xavier point of view i think that having that experience made him more human helped him connect to feeling that I believe he would have lost in the war. So my question to everyone else is whether or not you see importance in including these flashbacks in this novel?