Joint ownership.
I'm not quite sure I understand your answer. If you're not married, you have to pay taxes on the transfer of property (I believe it's 11K a year taxfree and 650K lifetime or something like that). If you are married, you can transfer unlimited amounts of property between spouses. Also, when a spouse dies, the surviving spouse inherits everything tax-free.
This is not an issue of gay marriage per se because unmarried straight couples face the same tax issues. They do have a remedy available though, getting married :)
As far as I know with joint ownership you do not get the tax benefits. Again, this affects couples of any kind, not necessarily gay ones.