Okay, how about this, then...
Do a little googling about Kerry when he was toying with McCain for a vice-president. I saw one HELL of a lot of articles about how cool it would be to have a Republican VP as Kerry's running mate. Never saw one single one about him having to change parties to do it.
From an old New York Times article (no longer on line). Boldface mine:
WASHINGTON, May 14 — Despite weeks of steadfast rejections from Senator John McCain, some prominent Democrats are angling for him to run for vice president alongside Senator John Kerry of Massachusetts, creating a bipartisan ticket that they say would instantly transform the presidential race.
In every wet dream some desperate Democratic politicians had about this ticket, the fact that it was bipartisan was a major plus in their eyes.
Yet one more example of the old double standard.