Wendy,
Yes, I am aware that some countries have changed the definition of marriage to include same-sex marriage.
Specifically, I was referring to the fact that in those countries, the society no longer has an institution which promotes a specific behavior (unite with a member of the opposite sex in order to create a family unit and raise children). Yes, there is still marriage, and yes, women and men can still marry and procreate. But the society is not *specifically promoting* that behavior anymore. The behavior society is promoting is something along the lines of "Find someone you love to live with".
As far as "living in sin", it is not clear what the effect will be. Some preliminary reports indicate the incidence of co-habitation actually increases in those countries which have allowed same-sex marriage, although the numbers are not conclusive.
Of course, if you take that phrase in its literal sense, same-sex couples are living in God-defined (or "religion-defined", if that's the way you look at it) sin regardless of their marital status, but I'm assuming that's not what you meant.
