Q: why morals are usually related to girls ?
A: Morals are culturally based. People think morals are related to girls because it's based on ignorance.
Q: I want to know where is the line?
Is it offensive for women to be bare chest if yes then why if no then why?
A: No. In some cultures, women do not cover their breasts. In some cultures like in Japan, men, women and children bathe together in public baths.
Q: does having multiple partner in case of girls is wrong or right?
A: It would be wrong, because children need fathers to help raise the child. If one woman had many children from many men, it becomes complicated because of parental responsibilities, legal issues, and confusion for the public at large.