The breasts of a female-bodied person are, biologically, not a primary sex organ. Boobs are meant to provide milk to babies; dogs don't have to cover up when breast feeding their pups, so why should women have to?
Like wise, although breasts are considered a source of aesthetic and/or sensual attraction, they are no more erotic than abs, a strong jawline, ect. Our culture has brainwashed us into assuming that it is the responsibility of women to not be a "turn on" to men in any way, thus enforcing misogyny.
Allowing boobs to be seen in public is more than just not wanting to wear shirts on hot days. Its not about wanting to be promiscuous, either. Allowing female-bodied people to be bare-chested is about desexualizing boobs, and enforcing equality between all of the sexes.