I ask this as a serious exercise in allegory to gay rights:

If I want to marry a gerbil, or a horse, or my right fist, do I have a constitutional right to that, and my freedoms are restricted if I'm not granted this right?

Or would such a legal union be a parody and an insult to true marriage, warping the definition of marriage beyond what is consistent and reasonable?

I could as easily argue that I should be able to marry a 12 year old girl, or a 10 year old girl, because it has previously been done, when human life expectancy averaged about 35 years (for most of human history), and therefore things like childhood and adolescence had to be skipped, in order to get on with the shorter business of life at that time.