I don’t think the Rambam says it’s a chiyuv because of chashad. I think he just says it’s a good idea.
Personally, I always had trouble understanding that Rambam (not to imply that my issues understanding the Rambam have any bearing on halacha, chas v’shalom, or on the truth of his words!). I have never noticed that there is a problem with chashad by older single girls, but that is a problem with older single boys.