It's not marketed specifically to boys though... Sure, some of the sets are obviously more male-oriented (perhaps even the majority), but there are also sets that are either specifically designed to appeal to girls, or which appeal more to girls for whatever reason.
This trend is really nothing new, either. More than 20 years ago I wanted the Lego spaceships, and my sister got pissed off at me for making wars against her happy little Lego town people. Even 20 years ago there were more sets that appealed to boys than girls, I'm reasonably sure.
We also had a pile of the plain old "gender-neutral" bricks, which either of us might have employed to augment our various constructions.