The word beau means fashionable, beautiful, gorgeous, handsome or really likable or lovable.
A female beau is a female who is gorgeous, beautiful and very lovely.
So if a man refers to you as a female beau then the man is complimenting you and saying you're are very gorgeous, likable, lovely and beautiful.
A male beau is a handsome, lovely, good looking male and a female who says that to a male is also complimenting the male.
Being called a beau is a good thing.
The word beau is a very old word and saying and is just a nice way of saying someone is very lovely.
For example I could also say my sons are beaus which means they are lovely and handsome and beautiful.