Given this, it's impossible to say that two men marrying is "the same as" a heterosexual couple or a lesbian couple. Indeed, you have to pull your lens out pretty far and ignore some pretty major differences in behavior patterns to declare similarity.

I'm not sure many people are claiming there are no differences between the sexes or that these differences are based only on discrimination. The question is whether being a man or a woman is relevant to the rights and responsibilities of the institution at hand.

Marriage is not the NBA. Marriage is not prison (altho many would beg to differ but that's another post). Marriage is not a public restroom. Marriage, as society understands it now, is an institution that protects and secures the rights and responsibilities of those with whom we form intimate partnerships.

Given this understanding of marriage, is there a sufficient reason to declare that only marriage between an adult male and an adult female is legal and valid? Or is there cause to argue that the sex of the individuals who wish to be married is irrelevant to the rights and responsibilities they are owed for participating?

I can understand your reasoning about not wishing to choose sides in a culture war, Xando.. but I, personally am loathe to decide my politics and personal ethics based on how comfortable people are with an issue or not.