User blog comment:TheBlueSlug/Ending/@comment-9339850-20130725094714/@comment-1359322-20130725100826

If Issei doesn't end with a harem would be like why did the author write 15 Volumes to strengthen the relationship between Issei and the girls.

I understand why you feel that it should be one man one woman, since if it's a harem then it would be like Issei is monopolizing the girls and making the other guys seem unimportant but then again anyone that has read the story from the start would feel that the girls deserve to be with Issei, especially the first four.