what makes you think that....?
all it will do is redefine marriage....
and how could you then deny marriage to a bisexual who wants to marry both a man and a woman?
or how could you deny a polygamist or polyamorist...? or other variations of 'marriage'....? once you open the door to redefine it the first time it will get redefined over and over again...
'open' marriages will become much more common.....if only because there are more people involved...or because gay men prefer it that way....
the whole concept of marriage and fidelity will change....
the institution of marriage will be torn asunder and become a patchwork of contracts....and lots more government instrusion...
what kind of effects do you think all that will have upon our children.....? confusion anyone? sexual identity problems? intolerance including against traditional homes? lost children in ever-changing homes? forced schooling against religious beliefs? and so on...
many will probably just get fed up even more with the whole concept of marriage as they are doing today.....pretty soon people will just live together like animals....eventually only a few will have "pedigrees".....

of course this all will make it easier for the STATE to step in and become the controlling influence...as we see already with many fatherless homes today...
It's bad enough now with no-fault divorce, the whole 'sexual revolution' and the resulting out-of-wedlock births....this will only throw more fuel on the fire that is destroying marriage and the basic building block of civilization....the basic family unit...dad, mom, and chlld...