shibumiseeker
Grandmaster
If the argument is, as proposed, "Anyone who is able to give consent is permitted to marry," then the next logical question is, "Who does that include or exclude?" The definition of "consenting adults" isn't even consistent from state to state. To say that that one believes that changing the parameters of marriage endangers a important societal norm (even some gays believe that) is not the same as saying, "it's icky and I don't like it." That's a straw man that's been grafted onto the former position.
My primary argument is that the government should get out of the "who can marry" business completely. Anything else is just a distraction.
The secondary justifications as to why the government should prevent consenting adults from marrying are flawed and boil down to "I think it's icky and should not be allowed". I have yet to see ANY rational justifications as to why allowing two or more consenting adults will bring down society. Point in fact if one of the goals of marriage is to provide for greater stability in a family structure then restricting who can marry also makes no sense since marriage is a stabilizing influence on interpersonal relationships regardless of the gender of the principles. The only justifications I have EVER seen were along the lines of either "slippery slope" (very, very weak), or "I think it's icky." (also very weak).