In the battle between the bigots and the progressives, something is kicked around, and thats the idea of Civil Unions or Domestic Partnerships, offered up as a compromise. Giving as many benefits as possible to LGBTQIA families provided that they are not called marriages. There are different flavors with different benefits throughout the USA, but just like marriages, they do not extend past the state line. Some states have parity or recognition of civil partnerships and marriages from other states. However the violation of the full faith and credit clause prevents these legal unions from being recognizes USA-wide.
Now for your for your average LGBTQIA couple they are looking for 3 things from marriage, firstly all the legal benefits of marriage, secondly all the legal responsibilities, and thirdly the actual recognized state of being married.
Now probably the strongest example of a civil union is that in the UK where aside from the name, its a marriage, however thats a rare case in a progressive country.
If you work for a decent company in a decent state and you are in a domestic partnership you may not have to worry about the legal limitations of domestic partnerships, but in the real world, your spouse may not be entitled to visit you in hospital, they may not be able to share your medical benefits, and every year LGBTQIA couples are penalized because they cannot jointly file taxes. There are also many rights recognized universally throughout the US for married couples, including immigration benefits, inheritance tax loopholes and simple spousal privilege in court (the right to refuse to testify against your spouse). Just like any cheap knock off, its not as functional, reliable or worth the same as the genuine article, and fobbing us off with Civil Unions is an insult.
Now on the right, with groups like NOM, the ADF and other hate groups the problem is that they do too much. NOM in particular are guilty of this little fiction, they frequently claim that its about protecting marriage, but then promote bills that would ban civil partnerships as well.
You look at their efforts around the USA, which vary from fighting against states that give benefits to same sex partners, or dare to pass real civil partnerships.
The problem for them is anything that gives LGBTQIA couples rights is a problem for them, so of course they are going to fight against civil partnerships. “The queers are thinking above their station” is the meme they are trying to spread around.
While on the other hand, being too much for the bigots, not only are civil partnerships separate but equal, quite often they do not provide all the rights of marriage.
The 1898 Plessy vs Ferguson decision codified into law the idea that provided you gave african american people something that sounded the same as given to white people, you could make them use different train cars, different stores, and different rights.
We need marriage equality, our forebears in the civil rights movement would not have put up with this, we owe it to ourselves, and their memories to keep fighting the good fight.