Perhaps I'm missing something - how could donating to Prop 8 be in support of abolishing the marriage institution?

Full text:

Section I. Title

This measure shall be known and may be cited as the "California Marriage Protection Act."

Section 2. Article I. Section 7.5 is added to the California Constitution, to read:

Sec. 7.5. Only marriage between a man and a woman is valid or recognized in California.