Why would any rational person compare the holding of that opinion to Nazism, murder, incest, or slavery? I'll help you all here. No rational person would make such an odious comparison.
BobKay was making a point to counter your "Marriage is a creature of the state, rooted in culture" assertion. Bringing Nazism into it was probably a bad call -- it never helps any argument, regardless of whether or not it fits (and I'm not saying it does here). Let's focus on the slavery comparison, instead.
Do you disagree that slavery was once a creature of the state? Do you disagree that it was rooted in culture? I doubt it. Bob's point was that these qualities of an institution like marriage are not sufficient cause to preserve it. Just because we've always done it that way doesn't mean it should not be revisited.
Culture is an explanation for why/how things are in a given place and time. It is based neither in fact, nor in the immutably physical. The only thing that it does for certain is change.
