It seems to me that it is true that some people (male, female, or what have you) may be more retiring and more hesitant to jump in when there's a break. That's quite understandable, and one of the functions of a moderator (official or self-appointed) is to ask for somebody (sometimes it's ANYBODY????) to perform, or even "Mary Jane, we haven't heard from you. Do you have something for us?"
BUT!!!
As I see it, such a retiring person, assuming he/she has a number of blank periods available to slide into, has no legitimate complaint if he/she doesn't take advantage of the opportunity that's there. And on the flip side, if there's gobs of "dead air", as there so often is, the eager singer who is willing to jump into the breach and keep the music going cannot fairly be criticized.