Sexism does exist in LD, and in more then one form. First, judges do not like to see a guy "beating up" a girl. It's just a natural, and usually commendable, instinct.
Second, alot of guys don't want to "beat up" girls. I for one intentionally scale down my aggressiveness when debating a girl. This is partially because i don't want to come across as mean, but also because I don't like to aggressively rip a girl, even if its just her case.
I don't believe a solution exists. Other then getting judges to try and suppress their instincts, i can't see a viable way to stop it. Plus, most of the time, being disgruntled by a guy being aggessive toward a girl is a good thing.