I don't know. If my best buddy dies, I don't get his shit unless it's in a will.
I'm not sure what the relation is here. Is your best buddy the one you've dedicated your life to, have sexual relations with, raise children with, own a home with, etc, per the OP's hypothetical? We're talking about marital type relationships without contract of actual marriage. Your relationship with your buddy isn't affected regardless of whether marriage exists or not.
"Leaving the survivor destitute" is sort of weird. I thought working to make a living was the prevailing thought in these parts. Seems contradictory to this sense of entitlement.
So in the non-marriage world, if a man "partners" with a woman, and he plays breadwinner, and she stays home and raises their 3 kids, and he dies, but in his will completely divests her of any bequest, giving it all to his friend Roy instead, you're ok with that because the "partner" getting anything would be fulfilling a false sense of entitlement? In almost all jurisdictions I'm aware of, the law prevents one spouse from completely divesting the other of assets via a will. Usually the survivor is granted 1/3 or 1/2 automatically but there are varying formulations. Without the official recognition of their marriage, the partner would get nothing. In the alternate case of the decedent dying intestate (without a will), how do we know how much to give to his life-partner, what if there is a dispute as to who his partner is, does it all just go to his kids, etc. Courts around the country, at the request of the decedent's family, routinely strike down wills which leave large bequests to suriving partners in homosexual relationships under the claim of "undue influence". Can you imagine being in a relationship for 40 years and then after your partner dies a court rules that you have no standing to receive anything because your relationship has been decreed a sham? That remains the reality for many gay couples today.
So again, marriage is a relatively simple contract that clarifies the will of the parties with regard to hundreds of legal and societal issues, and its status as a religious institution is far secondary, or tertiary, to its necessity in a modern society with complex property rights.
Now if you want to give all couples the ability to procure the benefits of marriage but simply call it civil unions, that's fine, as long as that's the only state sanctioned contract that applies to everyone equally. The name isn't important, so long as it's the only name. There is no beneficial purpose to maintainting two identical institutions which grant the exact same rights.