Romeo speaks these words shortly after being banished.
He is upset because now he cannot be near Juliet. Carrion flies may touch her, but he cannot get near her. As a result, carrion flies can woo Juliet better than he can, since he cannot get as close to her as he can.
Carrion flies are not banished and can therefore get close to Juliet, that is why Romeo says he is less honored than the flies.
