Something being "redefined" over history doesn't mean anything and doesn't change it's true definition.
All you are saying here is that it's been interpreted in ways it previously wasn't, which is always subjective.
Ah, I see you're unfamiliar with language. Let me help.
re·de·fine
1: to define (as a concept) again : reformulate <had to redefine their terms>
2a : to reexamine or reevaluate especially with a view to change
2b : transform
Something being "redefined" changes its definition. There's no "true" definition that stays current as something is redefined; there's just the definition that's been changed.
That said, what does the historic definition of marriage matter in regards to this issue? Is your only argument against gay marriage that it wasn't done in the past? Equal rights for women wasn't done in the past either; should we go back to traditional values on that as well? Democratic government wasn't done much in the past either. Perhaps we should get rid of that as well? An appeal to tradition is a logical fallacy that presumes that because something has been done a certain way in the past, that way is preferable. If someone can come up with demonstrable evidence that the old way is not preferable, the appeal to tradition is moot.
Restricting gay marriage prevents homosexual partners from enjoying government benefits that straight couples have access to, including tax incentives, next of kin delineation rights, visitation rights, insurance and retirement package benefits, and numerous other rights afforded straight married couples. Why is it OK to give these additional rights to heterosexual couples but not homosexual couples?