Marriage is an institution the government should have no control over, and that includes regulating its definition and arbitrarily deciding what benefits to give marriage and which ones to give domestic partnerships. Most of the arguments in favor of Prop. 8 fail to recognize that a marriage is NOT equal in terms of benefits to a domestic partnership. Some of the more prominent differences include a lack of tax benefits and no protection for the transfer of benefits if the partner dies. Also, there fewer court protections meaning someone who is married does not have to testify against his/her spouse, but someone who has a domestic partnership will have to testify against his/her partner if told to do so.
For a comprehensive list of the differences between marriage and domestic partnership click
HERE to read them for yourself.
Since the two institutions are not equal, there is no legal argument anyone can provide to justify prohibiting homosexuals from marrying especially in light of the Constitution's clear message of equality under the law. Some may argue that a homosexual is being treated equal under the law since they can still marry the opposite sex, but that simply deflects the true point that letting the government decide which consenting adults can marry and which one cannot is simply not justified in any case.
Now even if a domestic partnership and a marriage were the same and different in name only, that would still violate the Constitution as the courts long ago struck down the notion of "separate but equal institutions" as "inherently unequal". If there exists an institution for a man and woman to enter into a certain binding legal contract, than a man and a man or a woman and a woman should have the right to enter into that same exact binding legal contract.
To arbitrarily draw lines in the sand due to morality smacks of soft paternalism in which the government subtly decreases a certain behavior through psychological warfare. An example of this is the surgeon general's warning on cigarettes, and another is the distinction between a marriage for a man and woman and a domestic partnership for two people of the same sex. Soft paternalism opens the door to hard paternalism as the behavior being stigmatized is further seen as unwanted and options to stop it are seen as more attractive. You can read the full article on soft paternalism and hard paternalism
HERE.
The best solution would be to take the word marriage out of the government's hands and let the states retain "civil unions" or "domestic partnerships" which would be equal for both homosexual and heterosexual couples. That way those who wish to deny the word marriage to homosexual couples will feel validated that the word has returned into the hands of the people, and those understanding the importance of defending the minority's rights from the tyranny of democracy will also feel validated. If a religion doesn't want to marry a homosexual couple, it won't be forced to, but under the law a homosexual couple would finally have the same exact equal right to a union/partnership as a heterosexual couple which is the only acceptable solution in a truly free society.