What are morals? When does an act become offensive? Where should we draw the line?

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.

Mainly agreed. But men, women and children bathe together in Japan? Are they families? Or just strangers?

Families bathe with strangers at public baths. Our family used to do this before WWII in Sacramento where I was born and grew up. We lived in an area considered J-town in Sacramento. The area had many Japanese businesses including a soda factory. That area has been completely redeveloped, and there's no resemblance to it today except for Crocker Art Gallery. Most of the buildings today are state government office buildings.
However, there's still "Old Sacramento" which we recently visited. Old buildings built during the late 19th century are still there, and they have covered wagons and people in dated costumes to make it feel like the mid-19th, earth 20th century.

Sorry for the tangent, but these things just pops into my mind.