I get what you're saying...but surely the man-woman union is naturally "elevated" (for want of a better word) above gay unions by the fact that only hetero relationships can produce children. That's the way we're built and it's outside our control. But marriage is an artificial, man-made arrangement, and so ought to be adaptable to the changing needs and standards of society. The argument that marriage is a religious institution does not hold because it is possible to get married in a registry office with no reference to religion whatsoever, and a secular marriage has the same social and legal status as a religious one.
Besides, if/when the technology exists for gay couples to have their own kids (actually I think this is already possible, although illegal, with women - or am I wrong?), would that put them on the same level as hetero couples? Moral and ethical decisions like whether or not gays can marry ought to be completely independent of technological advancement.