The "disrespect" towards Karate stems from two main things.
1) The overall quality of Karate has declined, especially in the US. People with poor skill representing any art makes the art itself look bad. One of the major factors that contributed to the decline was the influx in Karate's popularity.
2) The untrained and even those that train in other arts can be ignorant. They base their opinions on inaccurate information and the biased opinions of others.
I've found myself in situations where my friends(untrained) know I train in martial arts and they give me their opinion on it. I try and clear up any misconceptions that they may have at the same time acknowledging the poor representation. I don't bring it up in conversations, but there are times people find out I train. My advice would be to not be ashamed, don't bring it up but know how to clear up misconceptions. George St Pierre, Lyoto Machida, Chuck Liddell and many other Pro fighters have karate backgrounds and utilize it in their fights.