The idea that a body part having a sexual function is a good justification for banning it from being uncovered in public is dubious, and trying to spread a false narrative about breasts having no such function just so you can defend reasonable treatment of breasts without challenging this dubious social standard is fairly self-defeating.
Do most men get aroused at the sight of female breasts? Do most women get aroused when they have their breasts fondled/licked/etc? If the answer is yes, then female breasts are a sexual organ. This isn't even a matter of opinion.
I'm radical enough to say, "sure." Doesn't hurt me any.
Sadly, we live in a society where genitals (and acts of sex) are not typically tolerated in public. That sucks. And if you want to undo that, I'm with you. Sex in public!
But there's also a different standard here. Restraining your ability to display your testicles in public (for example, on Instagram) doesn't, to my knowledge, affect your capacity to get by in life.
You might not realize this (or maybe you do), but for poor or homeless women, the difference between being able to breastfeed in public (and in some cases, at work) is the difference between being able to breastfeed and not, which in turn is the difference between statistical likelihood for a number of favorable health outcomes and not.
There's no reason not to tolerate - in fact embrace - the display of breasts and breastfeeding in socially normal places. It is a huge help to many women.