Its not that the Left and homosexuals disagree with Prop. 8 (though, of course, we do - I'm not saying we don't) - its that Prop. 8 takes away equal rights from people. The widely accepted Maslow's Heirarchy of Needs shows Love and Acceptance just above safety and security and the physiological needs of food, water, and shelter that are necessary for human survival.
IPT Bldg. 1 -- The Business Case for IPTs [Federal Knowledge Management Working Group (KMWG)]
Straight people can get married and receive all of the government bestowed benefits that help and assist them to stay with the person they love or, at least, the person they are accepted by and with which they chose to spend their lives. These benefits are not bestowed equally under civil unions.
Civil Unions vs. Gay Marriage
Freedom To Marry | Marriage vs Other
Civil Unions Vs Marriage In US: American LGBT Citizens Seek More Rights States Provide
Civil Unions vs Civil Marriage
The Left and homosexuals believe that everyone who is a US citizen deserves freedom and equality:
The Declaration of Independence
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
To survive human beings need fove, to be free human beings need equality, and for happiness human beings need love and freedom.
Homosexuals are human beings.
The reason Prop. 8 should be overturned isn't because the Left and homosexuals disagree with it, its because its wrong! In the US we bestow equal rights to people (except convicted felons) and any race, ethnicity, gender, religious affiliation, or creed. Those are qualities that can not be "changed" just like homosexuality. It isn't chosen. Homosexuals are gay because they are - that's it.
Slavery and discrimination against black people happened and was sanctioned by voter majorities but that doesn't make it right.
Prop. 8 should be overturned because it goes against the very ideals that this nation was founded on!