However, you're argument then becomes one of semantics. I will use opposite and same sex marriage to describe things to you, since you are insisting.
The government has good reasons to be in the civil marriage business. It has already been shown in this thread and others how marriage is beneficial to society as a whole. I believe CC posted some of the argument just within a few pages of this one. It also helps to protect individuals from abuses related to family structure and inheritance, and makes it so that a couple only has to make one contract instead of several to resolve legal issues that affect most couples, such as those I have mentioned. Most people who get married, want their choosen spouse to be the one to make those legal/medical decisions for them. Those who marry for some other reason, and want someone else to make such decisions, can designate the difference through their own legal contracts.
The spousal privilege laws are considered necessary for marriage. I believe they are considered along the same lines as attorney/client privileges and doctor/patient privileges. The difference is that it is assumed that someone will tell their spouse things they would not say to anyone else, because of their intimacy level. It is really a privacy issue.
Spousal privilege - Wikipedia, the free encyclopedia
Now, since you wish to make a distinction, the government assumes that most people want to be married due to an intimate connection. Because of this intimate connection, the couple wishes to make each other legal family. Now this is not always true. In fact, some opposite sex marriages have been merely smoke screens to hide one or both partners' homosexuality, especially during times when there were laws against homosexuality or when it may have been damaging to the person's career to come out.
I would love to know what these "goodies" of marriage are. Now certainly, there are some tax benefits to being married, but there are tax benefits to plenty of things that are seen as a benefit to society, such as charity and taking care of family members who do not/cannot take full care of themselves for whatever reason. The decision-making part is as much of a responsibility as a "goody", and can be designated to someone besides a spouse. The private businesses make their own rules on what "goodies" are given to spouses. Some businesses will even give those "goodies" to partners or whoever their worker wishes to designate.