symmetry is the default state of development, with consistent and functional (non-pathological) asymmetry requiring much more information, extra genetic material and many more coherent and viable evolutionary events.
Could you elaborate on that? And preferably in a more layman's language.
Embryonic development - the growth of the form - is managed by evolved genetic code in feedback with a given environment. Given an environment, simple code produces symmetry - the same rule followed the same way at all the growth locations will produce matching structures at the equivalent times and locations: symmetry, from the observer's point of view.
To produce an asymmetrical structure there must be more information supplied; usually, if not supplied by accident and mishap and incoherent feedback from the embryonic environment, that would mean auxiliary or additional or modified genetic code that acts to identify the locations separately and manages their development differently as the embryo grows (note that every cell must carry and properly employ all this code).
Symmetry is "easier", is all. Less code, less complexity, fewer possible problems in development, fewer successful mutations and evolutionary selection events to begin with, etc.
Given that basic situation, actual evolutionary selection for symmetry (sexual or otherwise) can arise, because most asymmetry would be accidental or pathological or otherwise evidence of weaker, less robust, more vulnerable genetic management - it would pay (in an evolutionary sense) to be able to recognize and (usually) avoid asymmetry in any situation in which benefit was to be had from well-functioning and robust genetic management.
In addition, sensory organs built as efficiently as possible (i.e. mostly symmetrical) are almost certain to register symmetrical input as reinforcing and clarifying- as a stronger signal or one more easily registered at weaker levels, and one more easily filtered for noise (the noise is unlikely to match).