Given what we know about the chemical aspects of love and how it changes over time you can make a pretty solid case that marriage has no grounding in natural law. Human society is built around mutual protection and labor sharing. Marriage really comes from the need for groups to define people’s relationship to each other and stuff (my house, my sword, my wife, go get your own). A lot of human society is unnatural constructs to avoid conflicts and when they arise to settle them through something other than violence.
Dupanloup I, Pereira L, Bertorelle G, Calafell F, Prata MJ, Amorim A, Barbujani G (2003). “A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity”. J Mol Evol 57 (1): 85–97. doi:10.1007/s00239-003-2458-x. PMID 12962309
http://link.springer.com/article/10.1007%2Fs00239-003-2458-x