My suspicion is that the push for same sex “marriage” is because these couples want the same financial benefits as traditional married couples (1 man + 1 woman,) and MOST specifically for tax breaks, since many corporations and insurance companies extend insurance benefits to same sex partners of established couples, and there doesn’t seem to be any rule that they go through a ceremony or buy some kind of license. Just sharing the same address for a certain period of time will do.
As far as all the other things married couples can do, that is pretty limitless. You can, for example, appoint anyone at all as an heir, a health care proxy, purchase property jointly with anyone you choose, and you can even adopt a child as a single person, no partner needed. Lest we forget, you can even incorporate yourself and another person: A corporation, for the purposes of law, is considered “a person.”
Since the government is whining about a lack of incoming revenue, I’d think they’d want to make the marriage tax benefit as narrow as possible, rather than open it up to a couple of million people who wouldn’t otherwise be entitled to it.