It's not about "leaving people out" -- those benefits and legal restrictions you refer to are all available to gays, they just might have to work a little harder to get them (e.g., write a will).
What it is about, though, is destroying the institution of marriage...and family. Observe what government policy has done to the black family, for cryin' out loud. Perhaps not intentionally, but in effect -- single moms with multiple children by multiple fathers, locked in a cycle of dependency.
Re-writing the laws of marriage -- which have served society so well over the centuries -- will, in the end, have the exact same effect on all families.
Co-habitation will be all about sex...and benefits. Commitment, marriage, motherhood and fatherhood will become artifacts of the past...children will be an "inconvenience", left to the government to raise.
Gay marriage isn't about creating "equal rights" for gays. It's about destroying a society they are alienated from. If you can't recognize this is a cultural war, you're going to help lose it.
If you're so concerned about two-person hetero marriage, then outlaw divorce, single parenting. There's far more of that and it's far more destructive than any two gals or guys who want to marry each other.
This is an interesting concept. How do you propose gay, unmarried couples transfer unlimited funds between partners or inherit unlimited amounts of money without paying taxes? I don't see an obvious way of doing that through a will or other legal document.