I think I might have some cultural differences here, because I don't understand why you would start with "I am against marriage because" and list a bunch of stuff, and then be for civil unions, but with more rights, which to me is just the same thing.
But I'm from France, and over there marriage is a civil contract. People are free to have a religious ceremony as well, after the legal one, and many people do, but the part where you sign the contract is the relevant one. However you celebrate isn't really relevant.
To me, marriage is a form of civil union, and for some reason religious people use the same word for their religious union as well, but even then, the one that's recognised by law is the civil one.
I personally understand that some of the benefits given with marriages/civil unions are unfair (for instance about taxes. I'm not sure why getting married should change how you pay your taxes, if nothing else has changed) but I think some other benefits are only fair, such as inheritance and being able to visit someone in the hospital and make end of life decisions for them.
I also admit to using the legal benefits of marriage to my advantage, as being from a different continent means that to live with a partner, one has to go through a lengthy immigration process, and one of the ways is to get married (not that it doesn't make it lengthy. It's just easier in this economy than finding a job in another country and being able to prove that they couldn't hire someone from their own country instead of you).
But I would be in favour of this advantage that's given to married partners being removed, provided immigration laws stopped being so stupid in the first place
If someone wants to show up and pay you taxes, I'm not sure why you'd want to kick them out, especially when there is no limit on who can become a citizen through birth. If a woman can have 10 children and all of them be allowed to live and work and pay taxes, why couldn't the same woman have 2 children, and 8 people come from other countries 18 years later to live and work and pay taxes? The country gets the same benefits from the workers, AND they didn't have to pay in education and so on to help raise them until now.
Anyways, that has nothing to do with marriage. But basically, I wouldn't be against the state removing every benefit, and it being a contract simply between the parties involved, where everyone sets their own rules (so no compulsory "you need to have sex" rule for instance).