Insensitive as his remark may be, Kerry's remark has a fair chunk of reasoning to back it up. In fact, I daresay that if you're posting on this forum, you almost certainly agree with him. The chief demographic of this message board seems to be college-age males, yet nobody that I know of has signed up for the USAF and is serving in Iraq.
Look at it this way: Kerry is speaking from the POV of somebody who is part of the group that gets to tell the people in Iraq what to do. Dropping out of school is probably not going to be conductive to getting a career as a politician. I would be interested in the statistics of education level of US Congressman, but I digress. Most people have a very healthy aversion to putting themselves in a situation where they're in danger of being shot at, blown up, etc etc. Some people are willing to serve on the basis that they feel that it is the right thing to do. But I'm sure that a number of other people join the USAF in order to secure themselves a better future. (I've thought about it)
Now from what I've heard, you do get at least some choice in your assignment when you join up. So the remark isn't literally true. But I think there's enough truth to the remark that it's had an effect, and that's part of the reason why the White House even bothered to respond to it. I would be much more amused if Kerry had made a remark about 'President Bush eats babies', but I doubt the white house would be demanding Kerry apologize to people's families in that case.