I don't think it's fair to call Senator Bernardi stupid, in my eyes, he made a valid point.
We have to remember that marriage is an arbitrary concept, it's not something which is natural, it is a man-made concept. Thus, by nature, marriage is what we, ourselves, define it to be. Some will define marriage as a union between a man and a woman who are not related, a union between two consenting people who are not related, a union between n consenting people...etc. So this links back to aabatery's argument, that disagreement and discrimination are two different things.
Someone who defines marriage as a union between a man and a woman does not necessarily discriminate against those who are gay. For example, one of my teachers last year (we had this debate in class), believed that a union between a man and a woman should be called marriage and a union between two of the same gender should be called a civil union. He agrees that under law, they should be given the same rights, but they should be called different things. Now, I don't think that's discriminatory, because he has clearly stated, that they should be given the same rights, he might just have a different definition of marriage to some others, who then might label him as a bigot, while he's really not.
Anecdote aside, I think this raises three views to the situation:
1) Marriage is between a man and a woman who are not related...etc. anything else should be called something different
2) Marriage is between any number of consenting people (or even animals), regardless of other factors
3) Those who are in between
The truth, in my opinion, is that most people fall into those who are in between and the the issue with that is, there's obviously SOMETHING which has made them move from group (1) closer to group (2). What Senator Bernardi is merely saying is that this could be an issue because that means that OTHER THINGS could possibly move them even MORE closer to group (2) - leading to things such as incest and marriage to animals...etc.
That doesn't necessarily make him a bigot, nor does that make him stupid, he hasn't said anything that is clearly discriminatory, he merely disagrees with those who wish to change the definition of marriage and he has every right to, especially in a country where we should be allowed to express our honest views.