Proposition 8 was not a campaign to change the political views of people. It had but one purpose, and that purpose was to declare a subset of the population legally unequal to everyone else.
Just so you know, the official slippery-slope argument is "I could marry my horse".
You anti-minor-marriage bigot you.
(To be clear, this is sarcasm hopefully in the service of making a point. I am not arguing for marriage between adults and minors.)
So, what about a post-menopausal mother marrying her adult son? If "consenting adults" is the standard, it must be sanctioned as "marriage," yes?
For the record, I'm not really against gay marriage, but I am staunchly against inconsistent reasoning in service of realpolitik.