I live in MS, and have yet to see a woman riding in less than a tri top. I wear a sleeveless jersey during the summer, but have been known to unzip it completely when riding alone in extreme heat. I am rather top heavy so I'd never wear only a sports bra, but wish I felt comfortable doing it at times. As for the sunscreen..........it is best applied 15-20 minutes before sun exposure for optimal protection. It's on the package.
Remember to each his/her own. Sometimes it's the person who is "offended" that needs to step away.