What are some of the reasons for which mainstream non-Catholic Christians are against gender-neutral secular marriage? What are some of the reasons for which mainstream non-Catholic Christians are against gender-neutral secular marriage?

I get that some churches won't acknowledge religious gender-neutral or same-sex marriage (GNM) and why they won't support GNM,

but why exactly does that extend to being outright against (as opposed to having no stand) secular GNM?

Besides homophobia (like with the Westboro Baptist Church and the sort) and possibly the arguments here, what are there?

In case the article is too long for you, here is a response that takes the article apart.

I guess mainstream is:

Orthodox (e.g. Eastern Orthodox, Greek Orthodox, Russian Orthodox, etc...)

Roman Catholic

Protestant (Lutheran, Baptist, Methodist, Episcopalian, etc...)

Historic Non-Chalcedonian Churches (Coptic, Armenian, Ethiopian, Nestorian, etc..)

Jehovah's Witness

The Church of Jesus Christ of Latter Day Saints (Mormons)

Seventh-Day Adventists
Christians believe that God's law is not an arbitrary set of rules designed to make life inconvenient, but that it represents God's intended pattern for living a happy, successful, fulfilled life. 

Many Christians believe that following God's pattern for human behaviour therefore results in more fulfilled people and a better society, whether or not those people believe in the source of the pattern (God). Many also believe that it is the duty of secular authorities to  promote moral behaviour; and especially the duty of Christians to use whatever influence they have on the government to make it promote moral behaviour.