One Man/One Woman, Is vs. Ought

Proponents of one-man+one-woman (1M+1W) choose to use an “is” phrasing, to try and suggest some forgone conclusion of what they really intend to be an “ought” issue. Instead of, “marriage should be between one man and one woman only,” they opt for, “marriage is defined as between one man and one woman [only].” The choice of “is” over “ought” phrasing indicates they at least subconsciously recognize the weak if not wholly untenable nature of their position, as prescriptions are usually more easily assailable than descriptions. It's one step away from such dishonest assertions, that can also be heard from some of the more shit-witted supporters of these measures, as “A gay man can still marry a woman,” à la “Let the lions eat salad!”… As if. That is admittedly on the harsher side of the spectrum, but the more common occurrence can be exemplified by a recent twitter exchange I came across, wherein one person relies on this tactic, as well as to use semantics to mitigate obvious consequences.

“How about two consenting adults?”
“You can argue that, I just don’t think that’s what marriage IS.”

This allows him to erroneously extend the description as a definition.

“Anti-gay marriage laws discriminate against people that are gay, no?”
“No, they define marriage as the union of one man and one woman.”

In other words, the respondent chooses to blinder himself against the direct consequent.

So again, this serves to illustrate an unconscious (I’m trying to be generous here), acknowledgement that the 1M+1W concept is basted in lamesauce, seeking to palliate that fact ever so slightly with a shift from prescription to description.

The verbiage takes a brief but convenient pause from the unspoken "ought," at the unfortunately standard wording of many of these legal propositions, written as "shall be defined," continuing swiftly to the "is" of spoken usage and talking points. The shift is easy to breeze past, even in multiple steps. They’re hoping you won’t catch it. Indeed, I’m sure some of them are hoping they themselves won’t catch it.

With regard to 1M+1W, not only has the "is" not been established, the "ought" has been refuted from every angle. 
Tradition! Bible! Mrrrca! - refuted. 
Optimal child-rearing! Slippery slope! Moral and social decay! -demolished.

Find a sewer pipe, 1M+1W, you’re on the crap side of history.