Please do not use tag wikis to define terms. That's not what the feature for. The blog post introducing the feature explains:
Tag wikis help introduce newcomers to the tag. They contain frequently asked questions about the tag and guidelines on its usage.
The "frequently asked questions" portion is best illustrated by large Stack Overflow tags such as C++ and PHP. More commonly, tag wikis explain how the tag should be used. A good example is catholicism on this site:
Use this tag for all questions about the organization of the Catholic church, its structures and practices and teachings specific to it, and of any other churches that fall within this definition.
Notice that the definition of Catholicism is for the purposes of organizing questions. The only reason to dive into controversial definitions is to help users decide if they are applying the proper tag(s).
Tag wikis should not be used as battlefields for controversial issues such as the definition of marriage. The current excerpt is more appropriate as the beginning of body of the tag and the body is more appropriate as the excerpt. So I've switched them up and expanded on the body. Please feel free to add additional details to the tag wiki, but please keep the excerpt focused on who the tag should be used on this site.
One of the great things about tags as organization tools is they usually don't need much explanation. When I use the heaven or hell tags, I can feel pretty confident it will help my question be properly categorized. This is true even though I mildly disagree with the tag wiki excerpts on this site. Folksonomies rely on people having the same idea of what terms mean as everyone else. Tags rarely require wikis to be used.
On occasion (especially on technical sites) terms mean something inscrutable to the average person not invested in the site's topic. (On this site, soteriology is a great example.) For those tags, the tag wiki is incredibly useful for helping people find the proper tag:
So is marriage such an easily defined term that we don't need a tag wiki? Or is it a technical term that needs to be defined explicitly? It's not as easy a question as you might think. On Saturdays, I go down to one of LA County's jails for men to teach a class on marriage and parenting. (This week, I'm going to be bringing the first lesson on marriage to the large group, as it turns out.) We stress the importance of making a life-long commitment to the mother of your children. Invariably, we get the question: "Which one?" because many of these men have children with multiple women. It's a question we simply cannot answer, so we politely decline to answer. (We suggest they take the question up with God in prayer.)
In terms of this site, if you think your question is about marriage, marriage is probably an appropriate tag. Since it's impossible to vote on tag wikis, they should strive for something like Wikipedia's neutral point of view.