It’s not about modesty.
Modesty isn’t defined by some recommended proportion of your body that should be covered by clothing. Rather, a large part of it is about not attracting undue attention to yourself. In this, the particular customs of place and time are obviously important. On a beach where most people go around 80% naked, Muslim women do not wear burkinis for reasons of modesty – to avoid attracting attention to themselves or to prevent lustful responses from those looking at them. They wear burkinis because they want to identify as Muslim or are frightened at what their community will do to them if they do not identify as Muslim.
…There really isn’t any middle-ground here. Either you think Islam, taken on its own and as a whole, is a threat to free civilization, or you do not. If you do not, then any legislation targeting Islam or Muslims is unjust. But if you do, then something must be done. While a burkini ban alone is obviously not that something, it may be part of a start.
And we’re past the point of innocent protestations – “I’m against terrorism, but people should be perfectly free to practice their own religion, including Islam if they so choose.”