Sex objects are noticed because of their body, and not their ideas, opinions, or talents, if only for that moment. Most likely all of us have objectified, or been objectified at some point in our lives.
With the internet, take the same situation, only instead of someone even seeing what you look like to objectify you, its done based solely on the implied gender. Being a female geek, I tend to be noticed for my gender, rather than my ideas or accomplishments.