Traditions change over time, the time for change has come, since we are now societally at a level advanced enough to recognize that when we say that everybody is equal before the law we actually should mean it.
Two elderly sisters marrying is fine with me, it doesn't bother me in the least what two elderly sisters are up to.
A mother marrying her son has connotations that society will probably never be comfortable with, even though in some countries it actually is legal.
A man marrying his horse is a funny example:
http://en.wikipedia.org/wiki/Human-animal_marriage
But personally I'd probably limit it to transactions between consenting humans.
My personal opinion on all this is that consenting people should be free to do whatever they want as long as it does not directly harm others.
Mental harm because they 'degrade' what you can do by doing it too does not count.