Marriage was an institution that ensured systematic oppression of half the population.The origin and evolution of marriage is discussed by Engels,using theories of Morgan, in his The Origin of the Family, Private Property & the State.
Sapiens also talk about evolution of family.

Marriages overtime led to deals by families so as to ensure wealth creation and maximisation, much like modern arranged marriages.
Homo sapiens did not evolve as monogamous, we institutionalised and conditioned monogamy, not for sanctity or morality but for property.

With agriculture and settled life, children were the early source of labour. An institution needed to be in place so as to ensure labour power for private property and wealth creation. Unions had to be made so as wealth stayed with the 'family' and more labour was generated

Union between two consenting individuals was not opposed by Marx or Engels as long as both individuals were respected and considered individuals with equal rights and freedom.
Union for wealth creation and oppression of one over the other was opposed and definitely needs to be.

Marriage was not a natural evolution. It was a system created for the above mentioned reasons and beyond. Honestly, monogamy in itself isn't natural for us and don't try the "we evolved into monogamous creatures", we didn't. We institutionalised monogamy, in favour of men.

Finally, people assuming that marriage in our society is the choice of individuals, men and women, you're nothing but privileged m*rons.

Choice is a privilege!