Marriage is a contract between two people, from which some people can be excluded b/c of gender. Two people can marry, unless their gender is the same. It's so silly. Actually, silly diminishes the wrong done. It's most definitely a civil rights and equality issue. Nobody is changing the definition, it's not a new contract being defined, it's simply eliminating exclusion b/c it's two people of the same gender rather than two people of different genders.
It's still a simple, straight-up, two-party contract.