what someone should tell the mormons

it strikes me that there is a point of law the gay people in california should explain to the mormon church.
of course, i voted against proposition 8. i don't much care what consenting adults do with their sex organs, as long as they allow me the same freedom. and i don't understand why heterosexuals who are married feel that gay marriage would somehow undermine some aspect of their own marriages. but aside from that, i think someone should explain to the mormons that gay marriage is in their own interest.
no one argues that the folks who wrote the california constitution had in mind to legalize gay marriage; but the constitution has been interpreted to protect a certain zone of privacy, in which such matters as procreation, contraception, sex between consenting adults, and marriage are protected from state interference without some compelling reason. gays argue, and the california supreme court has agreed, that this protected zone covers their activities, even though those were never contemplated by the writers of the constitution.
logically, since we are dealing with matters the makers of the constitution did not necessarily contemplate, why should this "zone of privacy," if it applies not only to the traditional man and woman but to man and man, not equally apply to other reasonable choices, such as a threesome? other than for the sake of religion, which is separated from government, or from tradition, which has been overcome in the decision on gays, why should marriage be limited to two persons, rather than three?
this is the point the mormons miss. having believed passionately in polygamy, and even killed over the issue, and having changed their views only at the point of a gun, under threat of violence and imprisonment, why should the mormons not fund the gay-marriage forces liberally, then ask the government to take one more small but logical step, and permit polygamy?