Analogies are of limited usefulness when trying to change someones mind. If I say, "It's just like interracial marriage . . ." or someone else says, "It's just like adult incest . . .", you pretty much know where we stand. So the analogy ends up being little more than stating your opinion.
I think analogies are more helpful when we are trying to analyze our own thoughts. So, consider the following list of potential or actual changes to marriage laws:
* Allowing fourteen year-olds to marry with parental consent
* Allowing a white person to marry a black person
* Allowing an adult man to marry his adult sister
* Allowing people to divorce without stating a cause
* Allowing an adult man to marry two adult women
* Recognizing a year of continuous cohabitation as a marriage
Why are each of these like or unlike allowing an adult man to marry another adult man, or an adult woman to marry another adult woman? Does this give you insight into your position on recognizing the marriages of gays and lesbians?