Edwards had a young son that died. If Cheney or Bush wanted to go low they could mention his dead son in some argument about healthcare or whatever, as a counter to Cheney's gay daughter cheapshots. But they don't. Bush and Cheney have more class and compassion than to embarass and upset someone like that...
That's a different matter entirely. Mentioning someone's sexual orientation still is less taboo than bringing up the dead
Edit: I don't know how else to word that....