Care, nurture, protection and provision are essential obligations for marriage because of a married couples' immense potential and privilege to have and raise children. Throughout history where marriage was ignored, the affect on women and children was disastrous. In some parts of the world today where men are deeply irresponsible, and refuse to marry, women and their fatherless children are the victims and often end up in a poverty-cycle not of their own making. In pre-WW2 Soviet Russia, marriage was relegated to an era of ideology not compatible with Atheistic Communist Socialism. When Russia faced invasion it realised that it did not have its most basic social unit reinforced: the nuclear family consisting of married parents with their children. This led to a nation on the verge of societal collapse. Marriage was promptly reinstated and promoted for the greater good of Mother Russia so that they stood chance of resisting their invaders.
Marriage is something. It is something that simply cannot be redefined in much the same that a shape, a colour or number is something in particular and therefore not something else. It is desperate and triflingly petty to call this an offensive notion.