Because this is what's most likely happening here in the next years.
Just today, the Constitutional Court ruled that same-sex couples must be given
full adoption rights, not only stepchild adoption.
http://www.thelocal.at/20150114/austria-lifts-ban-on-same-sex-adoptionThis means we are now one step further than Germany when it comes to same-sex marriage and adotion, because Germany only allows stepchild adoption so far.
And while we only have civil unions here that are different (or discriminating) relative to the hetero marriage law in ca. 30 points, the gay rights activists have sat together with the SPÖVP government and asked them to streamline the civil union law with the hetero law over the next years.
So, eventually the civil union law probably offers all the same rights and duties as the hetero law, just that it's still called "civil union" and not "marriage".
If all the rights and duties are the same though, incl. adoption and fertilisation - would you accept the civil union law to be in effect "marriage", or not ?
What do you think ?(PS: Of couse, the Austrian Constitutional Court has turned out to be very progressive anyway on these issues recently, so they might as well get a gay marriage case soon and rule that the government must introduce it, because it's otherwise discriminating).