Cheney already has flip-flopped. He says he will support whatever Bush decides :

http://www.365gay.com/newscon04/01/011104cheney.htm

As a dee-vor-see of 14 years standing, how come every time I see that phrase 'sanctity of marriage', it types outa my keyboard as the 'skanktity of mirage' ????

I don't know if that qualifies him as having "flip-flopped". It's obvious that his personal opinion is that there should not be a constitutional amendment against gay marriage. Declaring that he won't speak out against Bush doesn't mean he has changed his mind: it's just politically advantageous to mute his criticism.

Why is it obvious? How do you know he wasn't just saying whatever was politically advantageous at the time in 2000? Remember, they were running as 'compassionate conservatives'. If he had denounced civil unions then, it may have blown their cover.

His daughter is a card-carrying lesbian activist. If he had "flip-flopped" he would have came out in favor of the amendment, which he distinctly has not done. Instead, he said he would support the prez (his boss) in "whatever decision he makes." Bush still considers himself to be a compassionate conservative, so your explanation doesn't work.

I see no defense for the "sanctity of marriage". Does this apply to every marriage between a man and woman, the first, last and every one in between? Does it apply when the marriage is the result of unplanned pregnancy? Is a marriage sanctified because vows were taken before god and adminstered by a pedophile priest? How long must a marriage last to be sanctified? People compete for marriage on reality TV shows! Is there even a sobriety test involved?
Frankly, I surprised gays and lesbians would want to validate thier relationships using this institution as a standard of recognition.
As for those that would limit marriage to heterosexuals, I think they are afraid that gay and lesbian couples will will have a higher success rate.