Same sex marriage overlooks the fragile nature of that unique bond between man and woman. It ignores human nature that left to itself, will revert to the human savage nature that desires multiple partners. So typical of Marxism is that it ignores human nature and takes away the social supports that human nature needs to make marriage a success. This is done in the guise of equality and working for the common good.
Basically, same-sex marriage violates the natural parent-child bond in every family, and the right of the family to protection by society and the state. It will discard the fundamental understanding that the family is based upon the heterosexual marriage unit. This human unit is the very foundation of society. Same sex-marriage will de-naturalize the family by rendering in their entirety familial relationships as ever changing expressions of law. For thousands of years there has been a universal understanding that the family is a small society (a sovereign state) with unique rights and responsibilities independent of the state. The imposing of same sex marriage will no longer be in line with that understanding. Instead, the family will become mere policy relationships, defined and imposed by the state. The family (this society) founded more immediately in nature—the proles (procreation or fruitfulness)—will no longer have the distinction of sexual difference, with its distinctive quality of generative power.
Some are more nearly monogamous. Some are totally promiscuous, and some are in between ~ people, in a state of nature, are quite tied down with the child raising thing ~ and are essentially monogamous except when conditions dictate otherwise ~ like a high adult death rate!