I do not believe that comics are inherently sexist. The current portrayals of characters in comics tends towards being very stereotypical towards the genders, but this does not mean it is inherent. When considering the role of gender in comics, it is important to put it in context of the time period in which the comic was written. In modern times in particular, comic stories are beginning to show more of a realistic set of relationships between the genders. So, while people may object to what they feel are sexist themes in comic books, I believe it is something which is improving with time.
When creating comics, writers seek to make their characters relatable to a large, contemporary general population. In order to do this, they have to use some stereotypes and other generally acknowledged themes to make superhuman characters and stories more identifiable to every day people. When it comes to gender roles, this often means that the men and women in comics are portrayed in a way that is standard for the time period. So, for much of the history of comics, this means that women were often viewed as the weaker counterpart to their male comrades, an idea exemplified by the various love interests and side kicks created throughout the decades. However, this holds true for relatively independent characters as well. Even though heroes like Wonder Woman and Black Canary were strong and independent, aspects of their stories which could be viewed as sexist still manage to creep in (i.e. the skimpy, form fitting costumes or their tendency towards using feminine charms against their male opponents). However, the creators of the stories were not trying to be sexist. They were simply drawing on the ideas and themes of the time when creating the characters.
In modern times, comics have shown an increasing trend towards more accurate representation of gender roles. Lois Lane is a more realistic, three dimensional character that adds emotional themes to the Superman stories, not just a simpering, dewy-eyed dame for the Man of Steel to rescue. Female sidekicks are being given more active roles and are being taken more seriously (as much as sidekicks ever are). And Wonder Woman is becoming an even greater hero in her own right, showing ever greater strength, independence, and quality of characters. As the times change comic creators are updating their ideas to appeal to a modern audience, one where gender roles may be portrayed in a more realistic light. Now, this doesn't mean the system is perfect. There are still stereotypes that exist, and there will always be differences in the roles and actions of men and women in comics, just as their are in life. But the overall theme is an encouraging one, and just goes to show that comics are not inherently sexists. They are the product of their times.