yes, marriage started with adam and eve. It’s a big part of jp2’s theology of the body. A lot of the questions you posit are answered there, or in such encyclicals as arcanum and humana vitae, or by apologists such as dr. Jennifer roback morse or jimmy akin, or frequently on radio shows like catholic answers, sheila liaugminas, teresa tomeo, al kresta, et al. Catholic sources and rationale for marriage are in no shortage, and the reasoning is quite sound and powerful. It is simply ignored.
But the crux of this discussion is that the church has a particular understanding of marriage at present, which is contradictory to that which the state labels. The state’s new definition is even alien to that marriage founded in the natural law from the beginning. So the question is, what are the pros and cons for dissociating with the state’s dogma on this matter.