What makes the female breast sexual enough they can't be topless like men?

Is it just the Nipple? If so why? I've seen plenty of art of women with no top or bra, but no nipples and it be uncensored. Is it just the fact that it's female breasts? Religious? I can walk around nipple and man titty free, it feels good when mine are interacted with. Why don't I have to cover them at the pool if nips are sexual?