sargentlard said:
The issue then comes down to two extra questions among others
<ul>
<li>Is marriage strictly a union of two consenting adults?
<li>Or, is marriage a union between two consenting adults as long as they appease society's norms?
</ul>
Marriage definitely has something to do with society. There are many societies around where western "no-no"s are the normal case. I just want to remind you that marriages consisting of more than two partners are possible (one man - x women; but also one woman - x men) or that marriages between 13-year olds also occur - such things are culturally defined.
If you look at all forms of marriage you will find that there is one unique feature: marriage partners have certain
obligations towards each other (depending on culture, but the core of it is material provision for each other), but also
towards society (here I am not sure: should I write "provision of a home to offspring" or "producing offspring"; however, there are also other obligations, for instance the inclusion of the partner's family into the equation, meaning caring for in-laws and therefore providing a safety net, etc.).
I would imagine that in former times marriage was NOT created to produce offspring (that's perfectly possible without marriage and the same applies to incest), but to develop a work-unit where each partner had a predisposition for a certain type of task; men doing more physical demanding work, women more intricate tasks. Such multi-task work-units were certainly very beneficial to everyone - hence the trend to institutionalize it.
Now: marriage between siblings and father-mother-children would be complete nonsense in a way, because by bond of family those people ideally care for each other already. When the father dies, the daughter inherits. Why marry?
Marriage between gays: if the partners complement each other (and that must be the case; otherwise they wouldn't be attracted to one another) why not?