i've wondered the same thing, kellie! why is that sin, so much worse to enter a marriage with?
and actually, the history of marriage started as a business contract to bind two families together (i'll give you the crop of this field, if you gice your daughter to my son, that kind of thing)...or an economic contract to secure bloodlines of the noble, etc...
so, the history of marriage, had very little to do with religion, as i understand it.
it's another antiquated law, IMO, that should be changed now that we have learned more about the human body, and that homosexuality is most likely (definitely, IMO) biological.