I don't think gay marriage is fundamentally different than traditional heterosexual marriage at this point. Marriage has already shifted itself to something fundamentally different, at least within most of the developed world. Don't kid yourself, marriage hasn't been around for "tens of thousands of years," as anti-gay-marriage proponents tend to argue. It's probably somewhere between 4000 and 5000. Still a hell of a long time, probably not a relevant difference. But 4000 years ago marriage had 2 primary purposes: to produce children, and to yield a social organization that would allow for all the needs required to support individuals and their children to be fulfilled. Marriage is now fundamentally different, even among heterosexual couples. It is driven by factors such as mutual attraction, romantic interest, psychological support, and companionship. Those things weren't important enough to factor in thousands of years ago, and were still afterthoughts even a few hundred years ago. But it's really a Maslow's hierarchy of needs thing - we no longer need spouses to make sure we have a consistent suppy of food, clothing, and shelter, so we can start to look for marriages that support higher-order needs. There is no difference between the capacity for these modern motivators of marriage between homosexual and heterosexual couples. People that argue homosexual marriage is fundamentally different because it can't result in offspring are deluding themselves. Do some 30-something women "settle" because they feel their biological clocks ticking? Sure. But they are a small minority, and the mere construct of "settling" for the purpose of rearing children clearly demonstrates the backseat child-rearing has truly taken in the general spouse selection process in today's world. And plenty of children are conceived out of wedlock. This CDC faststat page
clearly demonstrates that reality - over 40% of children are now born out of wedlock, and some studies suggest half are conceived out of wedlock. The median age at which American women first give birth is nearly a year younger than the median age of their first marriage.
Given that, in lieu of these facts, it is very difficult to argue that marriage is fundamentally oriented towards production and rearing of children - or even that this remains a significant consideration for a huge proportion of the American population - what makes heterosexual and homosexual marriages "fundamentally different?"