If a man has the right to marry a man...
 

Why can't a man legally marry two women, or two men?

Think of what the purpose is behind each of those scenarios. Those with same-gender attraction seek equality because they want to enjoy all of the same social acceptance, rights, and privileges that traditional couples have.

What is a person who wants multiple marital partners trying to achieve?

Probably the same things.

I think the slope gets real slippery real fast. What if someone wants to marry a sibling, or a son or daughter, or an animal? Once you break from the traditional man/wife thing, deciding where the new line will be gets real tough.

I don't understand your logical chain of thought here. Why is it that saying marriage is NOT one thing necessarily means it is everything else?

Saying marriage is not just a marriage between a man and a woman is not tantamount to saying marriage is all other relationships.

If marriage is defined as a state recognized relationship between two people (except as specifically noted by statute, such as marriage to a minor or to a close relative), that encompasses a man and a woman, a woman and a woman, and a man and a man. It also doesn't require that the state recognize a marriage to a little child or to a goat. How do you get from Point A to Point B on this one?

It's basically this: if the definition of marriage is going to change to make a certain constituency happy, why not change it to make all constituents happy? Or atleast more than just one certain type of constituency happy?

Marriage already changes to make its constituencies happy. In some states, marriage with a second cousin is ok. In some, first cousin marriages are alright. In others, they aren't. The age of consent for marriage varies from state to state.

The state already defines marriage. If your opinion is that any state action defining marriage opens the door to further state action defining marriage, then you should be opposed to all state action in that arena, right? I doubt very much that you are.

If the reasons for prohibiting certain forms of marriage are simply public policy reasons, then isn't there a reasonable argument that in the past marriage was only between a man and a woman based on antiquated ideas of the "good" of public policy and those ideas are subject to revision to include a man and a woman? Certainly a state could also determine that polygamy was appropriate (they also have in the past), but it doesn't necessarily follow that they must accept polygamy if they accept gay marriage.

Sure. I'm just saying different people will want the line drawn in different places, and I think we'd atleast need to hear everybody out. Where is the harm in that?

laws againt consanguinous marriage are ok because they are for the good of society. And society trumps the rights of those individuals who seek to marry. If you buy that, then gay marriage can also be argued as detrimental to society, or not adding to society, therefore also legal. You have to at least allow for the possibility, if you are going to still ban consanguinous marriage.

I wonder if we will get into a situation at times where brothers "marry" purely for tax reasons.

Based on what? What scientific evidence is there that gay marriage is bad for society (in comparison to heterosexual marriage)? Consanguinous relations can scientifically be shown to result in genetic defects in offspring.