How does sexual selection affect introverts?

Sexual selection is where members of a species choose who to mate with (and therefore have to compete with other members of the species). This can lead to exaggeration of the signals that are used for selection (eg. if large tail feathers are seen as a proxy for health, then  larger and larger feathers may be selected over generations leading to things like peacock's displays).

Not sure what the connection with introverts would be. 

I can tell you this. I'm an introvert, have been all my life, the ones I've had sexual relations with are ones I was attracted to and quite specific as in anyone who don't meet the qualities I desire don't cut it for me. Interestingly, they were each extroverted.