Damn son, you missed the most obvious reason of all.

Nature.


Plain and simple nature. We exist on this planet for the simple reason of sex between opposite genders. Without it, we'd cease to exist. Nature does not require marriage I admit but marriage DOES promote nature's intent. Thus promoting nature is always suppose to be the politically correct way to govern a nation.

So, the liberal stance should be to promote nature's will through legislation making marriage between a man and a woman.
the conservative stance would be on the fact they find it offensive, crude or gross, etc...

If you're really trying to devise a paper you really need to consider the entire NATURE aspect as an argument point. Furthermore, the argument of homosexuality being a choice (not nature) then you can argue that nobody with such a mental problem should be allowed marriage until it's proved they are of a healthy mind.

Seriously, if you want to write this paper please consider some of what I typed. It will show MUCH more thought went into it than what you've already written.

Edited to add- After reading your entire post it's obvious to me your professor cares nothing about an argument of an opposing side nearly as much as he/she cares about being on the right side of the argument. What you typed does not sound like you remotely tried to argue the other side of the issue but instead reinforce your own side.