Everyone is in a family. Honor your father and mother. Everyone has one of each. Not everyone replicates that by becoming a husband or wife. But the design is husband and wife, not simply two people who love each other.
If there are benefits attached to being in a civil union, all that will happen is that people will abuse it for the benefits, much like they already abuse liberal divorce laws to use marriage for immigration purposes without any intention of actually forming a marriage. College roommates can have their relationship called a civil union in order get health care benefits or whatever for both once one of them gets a job. Once the idea of the permanent, organic union of male and female is lost, the whole thing devolves into a pointless sham.
There is nothing comparable to the two becoming one in marriage. It is not just the male and female body parts; it involves the whole lives, time and possessions and everything. They literally form one flesh. They belong to one another. Two men cannot replicate that. Nor can three people. It is a unique aspect of creation. There is no point in codifying imitations. Let gay couples be gay couples, but why license it? They do not and cannot become one flesh.